Method and device for transmitting and receiving tracking reference signal in wireless communication system supporting unlicensed band

ABSTRACT

Disclosed are a TRS receiving method comprising: receiving, from a base station, information indicating that a TRS is transmitted in a subframe; receiving the TRS transmitted in the subframe; and performing tracking by using the TRS, and a terminal performing, in the subframe to which the TRS is transmitted, the demodulation of data or not transmitting an acknowledgement/negative ACK (ACK/NACK) for the data.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the National Phase of PCT/KR2016/008389 filed on Jul. 29, 2016, which claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application Nos. 62/201,079 filed on Aug. 04, 2015 and 62/204,412 filed on Aug. 12, 2015, all of which are hereby expressly incorporated by reference into the present application.

TECHNICAL FIELD

Following description relates to a wireless communication system supporting an unlicensed band, and more particularly, to a method of transmitting and receiving a tracking reference signal, a method of performing RRM (radio resource measurement), and an apparatus therefor.

BACKGROUND ART

Wireless access systems have been widely deployed to provide various types of communication services such as voice or data. In general, a wireless access system is a multiple access system that may support communication of multiple users by sharing available system resources (e.g., a bandwidth, transmission power, etc.). For example, multiple access systems include a Code Division Multiple Access (CDMA) system, a Frequency Division Multiple Access (FDMA) system, a Time Division Multiple Access (TDMA) system, an Orthogonal Frequency Division Multiple Access (OFDMA) system, a Single Carrier Frequency Division Multiple Access (SC-FDMA) system, and a multi carrier frequency division multiple access (MC-FDMA) system.

DISCLOSURE OF THE INVENTION Technical Tasks

An object of the present invention is to provide a method of efficiently transmitting and receiving data in a wireless communication system supporting an unlicensed band.

Another object of the present invention is to sufficiently provide a tracking reference signal for performing tracking to a user equipment prior to downlink data transmission.

The other object of the present invention is to make a user equipment perform RRM measurement and report a result of the RRM measurement to a base station in a situation that a sample for performing the RRM measurement is not sufficient.

The technical problems solved by the present invention are not limited to the above technical problems and other technical problems which are not described herein will become apparent to those skilled in the art from the following description.

Technical Solution

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method of receiving a TRS (tracking reference signal), which is received by a user equipment in a wireless communication system supporting an unlicensed band, includes the steps of receiving, from a base station, information indicating that a TRS is transmitted in a subframe, receiving the TRS transmitted in the subframe, and performing tracking using the TRS. In this case, the user equipment does not demodulate data or does not transmit ACK/NACK (acknowledgment/negative ACK) in the subframe in which the TRS is transmitted.

The subframe in which the TRS is transmitted may correspond to a subframe other than a subframe in which a DRS (discovery reference signal) is transmitted.

The TRS can be composed of at least one selected from the group consisting of a DRS, a DMRS (demodulation reference signal), and a downlink transmission preamble.

The information indicating that the TRS is transmitted can include UE-specifically transmitted DCI (downlink control information) or an RNTI (radio network temporary identifier) commonly assigned to user equipments connected with the base station.

The tracking is performed using TRSs equal to or greater than the prescribed number of TRSs satisfying a tracking requirement configured by the base station and can be performed using both a TRS received within a time section configured by the base station and a TRS received in the subframe.

If the user equipment determines that a tracking requirement for performing tracking is not satisfied, the method can further include the step of requesting TRS transmission to the base station.

If a tracking requirement is not satisfied, the subframe can include a secondary DMTC (discovery measurement timing configuration) configured with a period shorter than a period of a primary DMTC.

To further achieve these and other advantages and in accordance with the purpose of the present invention, according to a different embodiment, a user equipment receiving a TRS (tracking reference signal) in a wireless communication system supporting an unlicensed band includes a transmitter, a receiver, and a processor configured to operate in a manner of being connected with the transmitter and the receiver, the processor configured to receive, from a base station, information indicating that a TRS is transmitted in a subframe, the processor configured to receive the TRS transmitted in the subframe, the processor configured to perform tracking using the TRS. In this case, the user equipment demodulates data or does not transmit ACK/NACK (acknowledgment/negative ACK) in the subframe in which the TRS is transmitted.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

Advantageous Effects

As is apparent from the above description, the embodiments of the present invention have the following effects.

First of all, it is able to efficiently transmit and receive data in a wireless communication system supporting an unlicensed band.

Second, since it is able to easily satisfy a tracking requirement for estimating a property of a radio channel, it is able to smoothly transmit downlink data.

Third, it is able to make a user equipment report a measurement result to a base station according to a channel status even in a situation that a sample for performing RRM measurement is not sufficient.

The effects of the present invention are not limited to the above-described effects and other effects which are not described herein may be derived by those skilled in the art from the following description of the embodiments of the present invention. That is, effects which are not intended by the present invention may be derived by those skilled in the art from the embodiments of the present invention.

DESCRIPTION OF DRAWINGS

The effects of the present invention are not limited to the above-described effects and other effects which are not described herein may be derived by those skilled in the art from the following description of the embodiments of the present invention. That is, effects which are not intended by the present invention may be derived by those skilled in the art from the embodiments of the present invention.

FIG. 1 is a diagram illustrating physical channels and a signal transmission method using the physical channels;

FIG. 2 is a diagram illustrating exemplary radio frame structures;

FIG. 3 is a diagram illustrating an exemplary resource grid for the duration of a downlink slot;

FIG. 4 is a diagram illustrating an exemplary structure of an uplink subframe;

FIG. 5 is a diagram illustrating an exemplary structure of a downlink subframe;

FIG. 6 is a view illustrating Physical Uplink Control Channel (PUCCH) formats 1a and 1b in a normal Cyclic Prefix (CP) case, and FIG. 7 is a view illustrating PUCCH formats 1a and 1b in an extended CP case;

FIG. 8 is a view illustrating PUCCH format 2/2a/2b in the normal CP case, and FIG. 9 is a view illustrating PUCCH format 2/2a/2b in the extended CP case;

FIG. 10 is a view illustrating Acknowledgment/Negative Acknowledgment (ACK/NACK) channelization for PUCCH formats 1a an 1b;

FIG. 11 is a view illustrating channelization for a hybrid structure of PUCCH format 1a/1b and PUCCH format 2/2a/2b in the same Physical Resource Block (PRB);

FIG. 12 is a view illustrating a PRB allocation method;

FIG. 13 is a view illustrating exemplary Component Carriers (CCs) and exemplary Carrier Aggregation (CA) in a Long Term Evolution-Advanced (LTE-A) system, which are used in embodiments of the present disclosure;

FIG. 14 is a view illustrating a subframe structure based on cross-carrier scheduling in the LTE-A system, which is used in embodiments of the present disclosure;

FIG. 15 is a view illustrating an exemplary configuration of serving cells according to cross-carrier scheduling used in embodiments of the present disclosure;

FIG. 16 is a view illustrating an exemplary new PUCCH format based on block spreading;

FIG. 17 is a view illustrating an exemplary configuration of a Resource Unit (RB) with time-frequency units;

FIG. 18 is a view illustrating an exemplary method for resource allocation and retransmission in asynchronous Hybrid Automatic Repeat reQuest (HARQ);

FIG. 19 is a conceptual view illustrating a Coordinated Multi-Point (CoMP) system operating in a CA environment;

FIG. 20 is a view illustrating an exemplary subframe to which User Equipment (UE)-specific Reference Signals (RSs) (UE-RSs) are allocated, which may be used in embodiments of the present disclosure;

FIG. 21 is a view illustrating an exemplary multiplexing of a legacy Physical Downlink Channel (PDCCH), a Physical Downlink Shared Channel (PDSCH), and an Enhanced PDCCH (E-PDCCH) in the LTE/LTE-A system;

FIG. 22 is a diagram illustrating an exemplary CA environment supported in an LTE-Unlicensed (LTE-U) system;

FIG. 23 is a diagram illustrating an exemplary Frame Based Equipment (FBE) operation as one of Listen-Before-Talk (LBT) operations;

FIG. 24 is a block diagram illustrating the FBE operation;

FIG. 25 is a diagram illustrating an exemplary Load Based Equipment (LBE) operation as one of the LBT operations;

FIG. 26 is a diagram for explaining DRS transmission methods supported by LAA system;

FIG. 27 is a flowchart for explaining CAP and CWA;

FIGS. 28 to 30 are diagrams illustrating a method of transmitting and receiving a TRS (tracking reference signal) according to a proposed embodiment;

FIG. 31 is a flowchart for a method of performing RRM (radio resource measurement) according to a proposed embodiment;

FIG. 32 is a diagram for configurations of a user equipment and a base station capable of implementing proposed embodiments.

BEST MODE

Mode for Invention

The following embodiments are proposed by combining constituent components and characteristics of the present invention according to a predetermined format. The individual constituent components or characteristics should be considered optional factors on the condition that there is no additional remark. If required, the individual constituent components or characteristics may not be combined with other components or characteristics. In addition, some constituent components and/or characteristics may be combined to implement the embodiments of the present invention. The order of operations to be disclosed in the embodiments of the present invention may be changed. Some components or characteristics of any embodiment may also be included in other embodiments, or may be replaced with those of the other embodiments as necessary.

In describing the present invention, if it is determined that the detailed description of a related known function or construction renders the scope of the present invention unnecessarily ambiguous, the detailed description thereof will be omitted.

In the entire specification, when a certain portion “comprises or includes” a certain component, this indicates that the other components are not excluded and may be further included unless specially described otherwise. The terms “unit”, “-or/er” and “module” described in the specification indicate a unit for processing at least one function or operation, which may be implemented by hardware, software or a combination thereof. The words “a or an”, “one”, “the” and words related thereto may be used to include both a singular expression and a plural expression unless the context describing the present invention (particularly, the context of the following claims) clearly indicates otherwise.

In the embodiments of the present disclosure, a description is mainly made of a data transmission and reception relationship between a Base Station (BS) and a User Equipment (UE). A BS refers to a terminal node of a network, which directly communicates with a UE. A specific operation described as being performed by the BS may be performed by an upper node of the BS.

Namely, it is apparent that, in a network comprised of a plurality of network nodes including a BS, various operations performed for communication with a UE may be performed by the BS, or network nodes other than the BS. The term ‘BS’ may be replaced with a fixed station, a Node B, an evolved Node B (eNode B or eNB), an Advanced Base Station (ABS), an access point, etc.

In the embodiments of the present disclosure, the term terminal may be replaced with a UE, a Mobile Station (MS), a Subscriber Station (SS), a Mobile Subscriber Station (MSS), a mobile terminal, an Advanced Mobile Station (AMS), etc.

A transmission end is a fixed and/or mobile node that provides a data service or a voice service and a reception end is a fixed and/or mobile node that receives a data service or a voice service. Therefore, a UE may serve as a transmission end and a BS may serve as a reception end, on an UpLink (UL). Likewise, the UE may serve as a reception end and the BS may serve as a transmission end, on a DownLink (DL).

The embodiments of the present disclosure may be supported by standard specifications disclosed for at least one of wireless access systems including an Institute of Electrical and Electronics Engineers (IEEE) 802.xx system, a 3rd Generation Partnership Project (3GPP) system, a 3GPP Long Term Evolution (LTE) system, and a 3GPP2 system. In particular, the embodiments of the present disclosure may be supported by the standard specifications, 3GPP TS 36.211, 3GPP TS 36.212, 3GPP TS 36.213, 3GPP TS 36.321 and 3GPP TS 36.331. That is, the steps or parts, which are not described to clearly reveal the technical idea of the present disclosure, in the embodiments of the present disclosure may be explained by the above standard specifications. All terms used in the embodiments of the present disclosure may be explained by the standard specifications.

Reference will now be made in detail to the embodiments of the present disclosure with reference to the accompanying drawings. The detailed description, which will be given below with reference to the accompanying drawings, is intended to explain exemplary embodiments of the present disclosure, rather than to show the only embodiments that can be implemented according to the disclosure.

The following detailed description includes specific terms in order to provide a thorough understanding of the present disclosure. However, it will be apparent to those skilled in the art that the specific terms may be replaced with other terms without departing the technical spirit and scope of the present disclosure.

For example, the term, TxOP may be used interchangeably with transmission period or Reserved Resource Period (RRP) in the same sense. Further, a Listen-Before-Talk (LBT) procedure may be performed for the same purpose as a carrier sensing procedure for determining whether a channel state is idle or busy.

Hereinafter, 3GPP LTE/LTE-A systems are explained, which are examples of wireless access systems.

The embodiments of the present disclosure can be applied to various wireless access systems such as Code Division Multiple Access (CDMA), Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Orthogonal Frequency Division Multiple Access (OFDMA), Single Carrier Frequency Division Multiple Access (SC-FDMA), etc.

CDMA may be implemented as a radio technology such as Universal Terrestrial Radio Access (UTRA) or CDMA2000. TDMA may be implemented as a radio technology such as Global System for Mobile communications (GSM)/General packet Radio Service (GPRS)/Enhanced Data Rates for GSM Evolution (EDGE). OFDMA may be implemented as a radio technology such as IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, Evolved UTRA (E-UTRA), etc.

UTRA is a part of Universal Mobile Telecommunications System (UMTS). 3GPP LTE is a part of Evolved UMTS (E-UMTS) using E-UTRA, adopting OFDMA for DL and SC-FDMA for UL. LTE-Advanced (LTE-A) is an evolution of 3GPP LTE. While the embodiments of the present disclosure are described in the context of a 3GPP LTE/LTE-A system in order to clarify the technical features of the present disclosure, the present disclosure is also applicable to an IEEE 802.16e/m system, etc.

1. 3GPP LTE/LTE-A System

In a wireless access system, a UE receives information from an eNB on a DL and transmits information to the eNB on a UL. The information transmitted and received between the UE and the eNB includes general data information and various types of control information. There are many physical channels according to the types/usages of information transmitted and received between the eNB and the UE.

1.1 Generals of System

FIG. 1 illustrates physical channels and a general signal transmission method using the physical channels, which may be used in embodiments of the present disclosure.

When a UE is powered on or enters a new cell, the UE performs initial cell search (S11). The initial cell search involves acquisition of synchronization to an eNB. Specifically, the UE synchronizes its timing to the eNB and acquires information such as a cell Identifier (ID) by receiving a Primary Synchronization Channel (P-SCH) and a Secondary Synchronization Channel (S-SCH) from the eNB.

Then the UE may acquire information broadcast in the cell by receiving a Physical Broadcast Channel (PBCH) from the eNB.

During the initial cell search, the UE may monitor a DL channel state by receiving a Downlink Reference Signal (DL RS).

After the initial cell search, the UE may acquire more detailed system information by receiving a Physical Downlink Control Channel (PDCCH) and receiving a Physical Downlink Shared Channel (PDSCH) based on information of the PDCCH (S12).

To complete connection to the eNB, the UE may perform a random access procedure with the eNB (S13 to S16). In the random access procedure, the UE may transmit a preamble on a Physical Random Access Channel (PRACH) (S13) and may receive a PDCCH and a PDSCH associated with the PDCCH (S14). In the case of contention-based random access, the UE may additionally perform a contention resolution procedure including transmission of an additional PRACH (S15) and reception of a PDCCH signal and a PDSCH signal corresponding to the PDCCH signal (S16).

After the above procedure, the UE may receive a PDCCH and/or a PDSCH from the eNB (S17) and transmit a Physical Uplink Shared Channel (PUSCH) and/or a Physical Uplink Control Channel (PUCCH) to the eNB (S18), in a general UL/DL signal transmission procedure.

Control information that the UE transmits to the eNB is generically called Uplink Control Information (UCI). The UCI includes a Hybrid Automatic Repeat and reQuest Acknowledgement/Negative Acknowledgement (HARQ-ACK/NACK), a Scheduling Request (SR), a Channel Quality Indicator (CQI), a Precoding Matrix Index (PMI), a Rank Indicator (RI), etc.

In the LTE system, UCI is generally transmitted on a PUCCH periodically. However, if control information and traffic data should be transmitted simultaneously, the control information and traffic data may be transmitted on a PUSCH. In addition, the UCI may be transmitted aperiodically on the PUSCH, upon receipt of a request/command from a network.

FIG. 2 illustrates exemplary radio frame structures used in embodiments of the present disclosure.

FIG. 2(a) illustrates frame structure type 1. Frame structure type 1 is applicable to both a full Frequency Division Duplex (FDD) system and a half FDD system.

One radio frame is 10 ms (Tf=307200·Ts) long, including equal-sized 20 slots indexed from 0 to 19. Each slot is 0.5 ms (Tslot=15360·Ts) long. One subframe includes two successive slots. An ith subframe includes 2ith and (2i+1)th slots. That is, a radio frame includes 10 subframes. A time required for transmitting one subframe is defined as a Transmission Time Interval (TTI). Ts is a sampling time given as Ts=1/(15 kHz×2048)=3.2552×10−8 (about 33 ns). One slot includes a plurality of Orthogonal Frequency Division Multiplexing (OFDM) symbols or SC-FDMA symbols in the time domain by a plurality of Resource Blocks (RBs) in the frequency domain.

A slot includes a plurality of OFDM symbols in the time domain. Since OFDMA is adopted for DL in the 3GPP LTE system, one OFDM symbol represents one symbol period. An OFDM symbol may be called an SC-FDMA symbol or symbol period. An RB is a resource allocation unit including a plurality of contiguous subcarriers in one slot.

In a full FDD system, each of 10 subframes may be used simultaneously for DL transmission and UL transmission during a 10-ms duration. The DL transmission and the UL transmission are distinguished by frequency. On the other hand, a UE cannot perform transmission and reception simultaneously in a half FDD system.

The above radio frame structure is purely exemplary. Thus, the number of subframes in a radio frame, the number of slots in a subframe, and the number of OFDM symbols in a slot may be changed.

FIG. 2(b) illustrates frame structure type 2. Frame structure type 2 is applied to a Time Division Duplex (TDD) system. One radio frame is 10 ms (Tf=307200·Ts) long, including two half-frames each having a length of 5 ms (=153600·Ts) long. Each half-frame includes five subframes each being 1 ms (=30720·Ts) long. An ith subframe includes 2ith and (2i+1)th slots each having a length of 0.5 ms (Tslot=15360·Ts). Ts is a sampling time given as Ts=1/(15 kHz×2048)=3.2552×10−8 (about 33 ns).

A type-2 frame includes a special subframe having three fields, Downlink Pilot Time Slot (DwPTS), Guard Period (GP), and Uplink Pilot Time Slot (UpPTS). The DwPTS is used for initial cell search, synchronization, or channel estimation at a UE, and the UpPTS is used for channel estimation and UL transmission synchronization with a UE at an eNB. The GP is used to cancel UL interference between a UL and a DL, caused by the multi-path delay of a DL signal.

[Table 1] below lists special subframe configurations (DwPTS/GP/UpPTS lengths).

TABLE 1 Normal cyclic prefix in downlink Extended cyclic prefix in downlink UpPTS upPTS Special Normal Extendend Normal Extended subframe cyclic prefix cyclic prefix cyclic prefix cyclic prefix configuration DwPTS in uplink in uplink DwPTS uplink uplink 0  6592 · T_(s) 2192 · T_(s) 2560 · T_(s)  7680 · T_(s) 2192 · T_(s) 2560 · T_(s) 1 19760 · T_(s) 20480 · T_(s) 2 21952 · T_(s) 23040 · T_(s) 3 24144 · T_(s) 25600 · T_(s) 4 26336 · T_(s)  7680 · T_(s) 4384 · T_(s) 5120 · T_(s) 5  6592 · T_(s) 4384 · T_(s) 5120 · T_(s) 20480 · T_(s) 6 19760 · T_(s) 23040 · T_(s) 7 21952 · T_(s) — — — 8 34144 · T_(s) — — —

FIG. 3 illustrates an exemplary structure of a DL resource grid for the duration of one DL slot, which may be used in embodiments of the present disclosure.

Referring to FIG. 3, a DL slot includes a plurality of OFDM symbols in the time domain. One DL slot includes 7 OFDM symbols in the time domain and an RB includes 12 subcarriers in the frequency domain, to which the present disclosure is not limited.

Each element of the resource grid is referred to as a Resource Element (RE). An RB includes 12×7 REs. The number of RBs in a DL slot, NDL depends on a DL transmission bandwidth. A UL slot may have the same structure as a DL slot.

FIG. 4 illustrates a structure of a UL subframe which may be used in embodiments of the present disclosure.

Referring to FIG. 4, a UL subframe may be divided into a control region and a data region in the frequency domain. A PUCCH carrying UCI is allocated to the control region and a PUSCH carrying user data is allocated to the data region. To maintain a single carrier property, a UE does not transmit a PUCCH and a PUSCH simultaneously. A pair of RBs in a subframe are allocated to a PUCCH for a UE. The RBs of the RB pair occupy different subcarriers in two slots. Thus it is said that the RB pair frequency-hops over a slot boundary.

FIG. 5 illustrates a structure of a DL subframe that may be used in embodiments of the present disclosure.

Referring to FIG. 5, up to three OFDM symbols of a DL subframe, starting from OFDM symbol 0 are used as a control region to which control channels are allocated and the other OFDM symbols of the DL subframe are used as a data region to which a PDSCH is allocated. DL control channels defined for the 3GPP LTE system include a Physical Control Format Indicator Channel (PCFICH), a PDCCH, and a Physical Hybrid ARQ Indicator Channel (PHICH).

The PCFICH is transmitted in the first OFDM symbol of a subframe, carrying information about the number of OFDM symbols used for transmission of control channels (i.e. the size of the control region) in the subframe. The PHICH is a response channel to a UL transmission, delivering an HARQ ACK/NACK signal. Control information carried on the PDCCH is called Downlink Control Information (DCI). The DCI transports UL resource assignment information, DL resource assignment information, or UL Transmission (Tx) power control commands for a UE group.

1.2 Physical Downlink Control Channel (PDCCH)

1.2.1 PDCCH Overview

The PDCCH may deliver information about resource allocation and a transport format for a Downlink Shared Channel (DL-SCH) (i.e. a DL grant), information about resource allocation and a transport format for an Uplink Shared Channel (UL-SCH) (i.e. a UL grant), paging information of a Paging Channel (PCH), system information on the DL-SCH, information about resource allocation for a higher-layer control message such as a random access response transmitted on the PDSCH, a set of Tx power control commands for individual UEs of a UE group, Voice Over Internet Protocol (VoIP) activation indication information, etc.

A plurality of PDCCHs may be transmitted in the control region. A UE may monitor a plurality of PDCCHs. A PDCCH is transmitted in an aggregate of one or more consecutive Control Channel Elements (CCEs). A PDCCH made up of one or more consecutive CCEs may be transmitted in the control region after subblock interleaving. A CCE is a logical allocation unit used to provide a PDCCH at a code rate based on the state of a radio channel. A CCE includes a plurality of RE Groups (REGs). The format of a PDCCH and the number of available bits for the PDCCH are determined according to the relationship between the number of CCEs and a code rate provided by the CCEs.

1.2.2 PDCCH Structure

A plurality of PDCCHs for a plurality of UEs may be multiplexed and transmitted in the control region. A PDCCH is made up of an aggregate of one or more consecutive CCEs. A CCE is a unit of 9 REGs each REG including 4 REs. Four Quadrature Phase Shift Keying (QPSK) symbols are mapped to each REG. REs occupied by RSs are excluded from REGs. That is, the total number of REGs in an OFDM symbol may be changed depending on the presence or absence of a cell-specific RS. The concept of an REG to which four REs are mapped is also applicable to other DL control channels (e.g. the PCFICH or the PHICH). Let the number of REGs that are not allocated to the PCFICH or the PHICH be denoted by NREG. Then the number of CCEs available to the system is NCCE (=└N_(REG)/9┘) and the CCEs are indexed from 0 to NCCE-1.

To simplify the decoding process of a UE, a PDCCH format including n CCEs may start with a CCE having an index equal to a multiple of n. That is, given CCE i, the PDCCH format may start with a CCE satisfying i mod n=0.

The eNB may configure a PDCCH with 1, 2, 4, or 8 CCEs. {1, 2, 4, 8} are called CCE aggregation levels. The number of CCEs used for transmission of a PDCCH is determined according to a channel state by the eNB. For example, one CCE is sufficient for a PDCCH directed to a UE in a good DL channel state (a UE near to the eNB). On the other hand, 8 CCEs may be required for a PDCCH directed to a UE in a poor DL channel state (a UE at a cell edge) in order to ensure sufficient robustness.

[Table 2] below illustrates PDCCH formats. 4 PDCCH formats are supported according to CCE aggregation levels as illustrated in [Table 2].

TABLE 2 PDCCH Number of Number of Number of format CCE (n) REG PDCCH bits 0 1 9 72 1 2 18 144 2 4 36 288 3 8 72 576

A different CCE aggregation level is allocated to each UE because the format or Modulation and Coding Scheme (MCS) level of control information delivered in a PDCCH for the UE is different. An MCS level defines a code rate used for data coding and a modulation order. An adaptive MCS level is used for link adaptation. In general, three or four MCS levels may be considered for control channels carrying control information.

Regarding the formats of control information, control information transmitted on a PDCCH is called DCI. The configuration of information in PDCCH payload may be changed depending on the DCI format. The PDCCH payload is information bits. [Table 3] lists DCI according to DCI formats.

TABLE 3 DCI Format Description Format Resource grants for PUSCH transmissions (uplink) 0 Format Resource assignments for single codeword PDSCH transmission 1 (transmission modes 1, 2 and 7) Format Compact signaling of resource assignments for single codeword 1A PDSCH (all modes) Format Compact resource assignments for PDSCH using rank-1 closed 1B loop precoding (mode 6) Format Very compact resource assignments for PDSCH (e.g., 1C paging/broadcast system information) Format Compact resource assignments for PDSCH using multi-user 1D MIMO(mode 5) Format Resource assignments for PDSCH for closed loop MIMO 2 operation (mode 4) Format resource assignments for PDSCH for open loop MIMO operation 2A (mode 3) Format Power control commands for PUCCH and PUSCH with 2-bit/1- 3/3A bit power adjustment Format Scheduling of PUSCH in one UL cell with multi-antenna port 4 transmission mode

Referring to [Table 3], the DCI formats include Format 0 for PUSCH scheduling, Format 1 for single-codeword PDSCH scheduling, Format 1A for compact single-codeword PDSCH scheduling, Format 1C for very compact DL-SCH scheduling, Format 2 for PDSCH scheduling in a closed-loop spatial multiplexing mode, Format 2A for PDSCH scheduling in an open-loop spatial multiplexing mode, and Format 3/3A for transmission of Transmission Power Control (TPC) commands for uplink channels. DCI Format 1A is available for PDSCH scheduling irrespective of the transmission mode of a UE.

The length of PDCCH payload may vary with DCI formats. In addition, the type and length of PDCCH payload may be changed depending on compact or non-compact scheduling or the transmission mode of a UE.

The transmission mode of a UE may be configured for DL data reception on a PDSCH at the UE. For example, DL data carried on a PDSCH includes scheduled data, a paging message, a random access response, broadcast information on a BCCH, etc. for a UE. The DL data of the PDSCH is related to a DCI format signaled through a PDCCH. The transmission mode may be configured semi-statically for the UE by higher-layer signaling (e.g. Radio Resource Control (RRC) signaling). The transmission mode may be classified as single antenna transmission or multi-antenna transmission.

A transmission mode is configured for a UE semi-statically by higher-layer signaling. For example, multi-antenna transmission scheme may include transmit diversity, open-loop or closed-loop spatial multiplexing, Multi-User Multiple Input Multiple Output (MU-MIMO), or beamforming. Transmit diversity increases transmission reliability by transmitting the same data through multiple Tx antennas. Spatial multiplexing enables high-speed data transmission without increasing a system bandwidth by simultaneously transmitting different data through multiple Tx antennas. Beamforming is a technique of increasing the Signal to Interference plus Noise Ratio (SINR) of a signal by weighting multiple antennas according to channel states.

A DCI format for a UE depends on the transmission mode of the UE. The UE has a reference DCI format monitored according to the transmission mode configure for the UE. The following 10 transmission modes are available to UEs:

(1) Transmission mode 1: Single antenna port (port 0);

(2) Transmission mode 2: Transmit diversity;

(3) Transmission mode 3: Open-loop spatial multiplexing when the number of layer is larger than 1 or Transmit diversity when the rank is 1;

(4) Transmission mode 4: Closed-loop spatial multiplexing;

(5) Transmission mode 5: MU-MIMO;

(6) Transmission mode 6: Closed-loop rank-1 precoding;

(7) Transmission mode 7: Precoding supporting a single layer transmission, which is not based on a codebook (Rel-8);

(8) Transmission mode 8: Precoding supporting up to two layers, which are not based on a codebook (Rel-9);

(9) Transmission mode 9: Precoding supporting up to eight layers, which are not based on a codebook (Rel-10); and

(10) Transmission mode 10: Precoding supporting up to eight layers, which are not based on a codebook, used for CoMP (Rel-11).

1.2.3 PDCCH Transmission

The eNB determines a PDCCH format according to DCI that will be transmitted to the UE and adds a Cyclic Redundancy Check (CRC) to the control information. The CRC is masked by a unique Identifier (ID) (e.g. a Radio Network Temporary Identifier (RNTI)) according to the owner or usage of the PDCCH. If the PDCCH is destined for a specific UE, the CRC may be masked by a unique ID (e.g. a cell-RNTI (C-RNTI)) of the UE. If the PDCCH carries a paging message, the CRC of the PDCCH may be masked by a paging indicator ID (e.g. a Paging-RNTI (P-RNTI)). If the PDCCH carries system information, particularly, a System Information Block (SIB), its CRC may be masked by a system information ID (e.g. a System Information RNTI (SI-RNTI)). To indicate that the PDCCH carries a random access response to a random access preamble transmitted by a UE, its CRC may be masked by a Random Access-RNTI (RA-RNTI).

Then, the eNB generates coded data by channel-encoding the CRC-added control information. The channel coding may be performed at a code rate corresponding to an MCS level. The eNB rate-matches the coded data according to a CCE aggregation level allocated to a PDCCH format and generates modulation symbols by modulating the coded data. Herein, a modulation order corresponding to the MCS level may be used for the modulation. The CCE aggregation level for the modulation symbols of a PDCCH may be one of 1, 2, 4, and 8. Subsequently, the eNB maps the modulation symbols to physical REs (i.e. CCE to RE mapping).

1.2.4 Blind Decoding (BD)

A plurality of PDCCHs may be transmitted in a subframe. That is, the control region of a subframe includes a plurality of CCEs, CCE 0 to CCE NCCE,k-1. NCCE,k is the total number of CCEs in the control region of a kth subframe. A UE monitors a plurality of PDCCHs in every subframe. This means that the UE attempts to decode each PDCCH according to a monitored PDCCH format.

The eNB does not provide the UE with information about the position of a PDCCH directed to the UE in an allocated control region of a subframe. Without knowledge of the position, CCE aggregation level, or DCI format of its PDCCH, the UE searches for its PDCCH by monitoring a set of PDCCH candidates in the subframe in order to receive a control channel from the eNB. This is called blind decoding. Blind decoding is the process of demasking a CRC part with a UE ID, checking a CRC error, and determining whether a corresponding PDCCH is a control channel directed to a UE by the UE.

The UE monitors a PDCCH in every subframe to receive data transmitted to the UE in an active mode. In a Discontinuous Reception (DRX) mode, the UE wakes up in a monitoring interval of every DRX cycle and monitors a PDCCH in a subframe corresponding to the monitoring interval. The PDCCH-monitored subframe is called a non-DRX subframe.

To receive its PDCCH, the UE should blind-decode all CCEs of the control region of the non-DRX subframe. Without knowledge of a transmitted PDCCH format, the UE should decode all PDCCHs with all possible CCE aggregation levels until the UE succeeds in blind-decoding a PDCCH in every non-DRX subframe. Since the UE does not know the number of CCEs used for its PDCCH, the UE should attempt detection with all possible CCE aggregation levels until the UE succeeds in blind decoding of a PDCCH.

In the LTE system, the concept of Search Space (SS) is defined for blind decoding of a UE. An SS is a set of PDCCH candidates that a UE will monitor. The SS may have a different size for each PDCCH format. There are two types of SSs, Common Search Space (CSS) and UE-specific/Dedicated Search Space (USS).

While all UEs may know the size of a CSS, a USS may be configured for each individual UE. Accordingly, a UE should monitor both a CSS and a USS to decode a PDCCH. As a consequence, the UE performs up to 44 blind decodings in one subframe, except for blind decodings based on different CRC values (e.g., C-RNTI, P-RNTI, SI-RNTI, and RA-RNTI).

In view of the constraints of an SS, the eNB may not secure CCE resources to transmit PDCCHs to all intended UEs in a given subframe. This situation occurs because the remaining resources except for allocated CCEs may not be included in an SS for a specific UE. To minimize this obstacle that may continue in the next subframe, a UE-specific hopping sequence may apply to the starting position of a USS.

[Table 4] illustrates the sizes of CSSs and USSs.

TABLE 4 PDCCH Number of Number of Number of Format CCE (n) candidates in CSS candidates in USS 0 1 — 6 1 2 — 6 2 4 4 2 3 8 2 2

To mitigate the load of the UE caused by the number of blind decoding attempts, the UE does not search for all defined DCI formats simultaneously. Specifically, the UE always searches for DCI Format 0 and DCI Format 1A in a USS. Although DCI Format 0 and DCI Format 1A are of the same size, the UE may distinguish the DCI formats by a flag for format 0/format 1a differentiation included in a PDCCH. Other DCI formats than DCI Format 0 and DCI Format 1A, such as DCI Format 1, DCI Format 1B, and DCI Format 2 may be required for the UE.

The UE may search for DCI Format 1A and DCI Format 1C in a CSS. The UE may also be configured to search for DCI Format 3 or 3A in the CSS. Although DCI Format 3 and DCI Format 3A have the same size as DCI Format 0 and DCI Format 1A, the UE may distinguish the DCI formats by a CRC scrambled with an ID other than a UE-specific ID.

An SS S_(k) ^((L)) is a PDCCH candidate set with a CCE aggregation level L∈{1, 2, 4, 8} The CCEs of PDCCH candidate set m in the SS may be determined by the following equation. L·{(Y_(k)+m)mod └N_(CCE,k)/L┘}+i  [Equation 1]

Herein, M^((L)) is the number of PDCCH candidates with CCE aggregation level L to be monitored in the SS, m=0, . . . , M^((L))−1, i is the index of a CCE in each PDCCH candidate, and i=0, . . . , L−1. k=└n_(s)/2┘ where n_(s) is the index of a slot in a radio frame.

As described before, the UE monitors both the USS and the CSS to decode a PDCCH. The CSS supports PDCCHs with CCE aggregation levels {4, 8} and the USS supports PDCCHs with CCE aggregation levels {1, 2, 4, 8}. [Table 5] illustrates PDCCH candidates monitored by a UE.

TABLE 5 Search space S_(k) ^((L)) Number of PDCCH Type Aggregation level L Size [in CCEs] candidates M^((L)) UE-specific 1 6 6 2 12 6 4 8 2 8 16 2 Common 4 16 4 8 16 2

Referring to [Equation 1], for two aggregation levels, L=4 and L=8, Y_(k) is set to 0 in the CSS, whereas Y_(k) is defined by [Equation 2] for aggregation level L in the USS. Y _(k)=(A·Y _(k−1))mod D  [Equation 2]

Herein, Y⁻¹=n_(RNTI)≠0, n_(RNTI) indicating an RNTI value. A=39827 and D=65537.

1.3. PUCCH (Physical Uplink Control Channel)

PUCCH may include the following formats to transmit control information.

(1) Format 1: On-Off keying (OOK) modulation, used for SR (Scheduling Request)

(2) Format 1a & 1b: Used for ACK/NACK transmission

-   -   1) Format 1a: BPSK ACK/NACK for 1 codeword     -   2) Format 1b: QPSK ACK/NACK for 2 codewords

(3) Format 2: QPSK modulation, used for CQI transmission

(4) Format 2a & Format 2b: Used for simultaneous transmission of CQI and ACK/NACK

(5) Format 3: Used for multiple ACK/NACK transmission in a carrier aggregation environment

[Table 6] shows a modulation scheme according to PUCCH format and the number of bits per subframe. Table 7 shows the number of reference signals (RS) per slot according to PUCCH format. Table 8 shows SC-FDMA symbol location of RS (reference signal) according to PUCCH format. In Table 6, PUCCH format 2a and PUCCH format 2b correspond to a case of normal cyclic prefix (CP).

TABLE 6 PUCCH format Modulation scheme No. of bits per subframe, Mbit 1 N/A N/A 1a BPSK 1 1b QPSK 2 2 QPSK 20 2a QPSK + BPSK 21 2b QPSK + BPSK 22 3 QPSK 48

TABLE 7 PUCCH format Normal CP Extended CP 1, 1a, 1b 3 2 2, 3 2 1 2a, 2b 2 N/A

TABLE 8 PUCCH SC-FDMA symbol location of RS format Normal CP Extended CP 1, 1a, 1b 2, 3, 4 2, 3 2, 3 1, 5 3 2a, 2b 1, 5 N/A

FIG. 6 shows PUCCH formats 1a and 1b in case of a normal cyclic prefix. And, FIG. 7 shows PUCCH formats 1a and 1b in case of an extended cyclic prefix.

According to the PUCCH formats 1a and 1b, control information of the same content is repeated in a subframe by slot unit. In each UE, ACK/NACK signal is transmitted on a different resource constructed with a different cyclic shift (CS) (frequency domain code) and an orthogonal cover (OC) or orthogonal cover code (OCC) (time domain spreading code) of CG-CAZAC (computer-generated constant amplitude zero auto correlation) sequence. For instance, the OC includes Walsh/DFT orthogonal code. If the number of CS and the number of OC are 6 and 3, respectively, total 18 UEs may be multiplexed within the same PRB (physical resource block) with reference to a single antenna. Orthogonal sequences w0, w1, w2 and w3 may be applicable to a random time domain (after FFT modulation) or a random frequency domain (before FFT modulation).

For persistent scheduling with SR, ACK/NACK resource constructed with CS, OC and PRB (physical resource block) may be allocated to a UE through RRC (radio resource control. For non-persistent scheduling with dynamic ACK/NACK, the ACK/NACK resource may be implicitly allocated to a UE using a smallest CCE index of PDCCH corresponding to PDSCH.

Length-4 orthogonal sequence (OC) and length-3 orthogonal sequence for PUCCH format 1/1a/1b are shown in Table 9 and Table 10, respectively.

TABLE 9 Sequence index Orthogonal sequences n_(oc) (n_(s)) [w(0) . . . w(N_(SF) ^(PUCCH) − 1)] 0 [+1 +1 +1 +1] 1 [+1 −1 +1 −1] 2 [+1 −1 −1 +1]

TABLE 10 Sequence index Orthogonal sequences n_(oc) (n_(s)) [w(0) . . . w(N_(SF) ^(PUCCH) − 1)] 0 [1 1 1] 1 [1 e^(j2π/3) e^(j4π/3)] 2 [1 e^(j4π/3) e^(j2π/3)]

Orthogonal sequence (OC) [w(0) . . . w(N_(RS) ^(PUCCH)−1)] for a reference signal in PUCCH format 1/1a/1b is shown in Table 11.

TABLE 11 Sequence index n _(oc) (n_(s)) Normal cyclic prefix Extended cyclic prefix 0 [1 1 1] [1 1] 1 [1 e^(j2π/3) e^(j4π/3)] [1 −1] 2 [1 e^(j4π/3) e^(j2π/3)] N/A

FIG. 8 shows PUCCH format 2/2a/2b in case of a normal cyclic prefix. And, FIG. 9 shows PUCCH format 2/2a/2b in case of an extended cyclic prefix.

Referring to FIG. 8 and FIG. 9, in case of a normal CP, a subframe is constructed with 10 QPSK data symbols as well as RS symbol. Each QPSK symbol is spread in a frequency domain by CS and is then mapped to a corresponding SC-FDMA symbol. SC-FDMA symbol level CS hopping may be applied to randomize inter-cell interference. The RS may be multiplexed by CDM using a cyclic shift. For instance, assuming that the number of available CSs is 12, 12 UEs may be multiplexed in the same PRB. For instance, assuming that the number of available CSs is 6, 6 UEs may be multiplexed in the same PRB. In brief, a plurality of UEs in PUCCH format 1/1a/1b and PUCCH format 2/2a/2b may be multiplexed by ‘CS+OC+PRB’ and ‘CS+PRB’, respectively.

FIG. 10 is a diagram of ACK/NACK channelization for PUCCH formats 1a and 1b. In particular, FIG. 10 corresponds to a case of ‘Δ_(shift) ^(PUCCH)=2’

FIG. 11 is a diagram of channelization for a hybrid structure of PUCCH format 1/1a/1b and PUCCH format 2/2a/2b.

Cyclic shift (CS) hopping and orthogonal cover (OC) remapping may be applicable in a following manner.

(1) Symbol-based cell-specific CS hopping for randomization of inter-cell interference

(2) Slot level CS/OC remapping

-   -   1) For inter-cell interference randomization     -   2) Slot based access for mapping between ACK/NACK channel and         resource (k)

Meanwhile, resource n_(r) for PUCCH format 1/1a/1b may include the following combinations.

(1) CS (=equal to DFT orthogonal code at symbol level) (n_(cs))

(2) OC (orthogonal cover at slot level) (n_(oc))

(3) Frequency RB (Resource Block) (n_(rb))

If indexes indicating CS, OC and RB are set to n_(cs), n_(oc), n_(rb), respectively, a representative index n_(r) may include n_(cs), n_(oc) and n_(rb). In this case, the n_(r) may meet the condition of ‘n_(r)=(n_(cs), n_(oc), n_(rb))’.

The combination of CQI, PMI, RI, CQI and ACK/NACK may be delivered through the PUCCH format 2/2a/2b. And, Reed Muller (RM) channel coding may be applicable.

For instance, channel coding for UL (uplink) CQI in LTE system may be described as follows. First of all, bitstreams a₀, a₁, a₂, a₃, . . . , a_(A−1) may be coded using (20, A) RM code. In this case, a_(α) and a_(A−1) indicates MSB (Most Significant Bit) and LSB (Least Significant Bit), respectively. In case of an extended cyclic prefix, maximum information bits include 11 bits except a case that QI and ACK/NACK are simultaneously transmitted. After coding has been performed with 20 bits using the RM code, QPSK modulation may be applied. Before the BPSK modulation, coded bits may be scrambled.

Table 12 shows a basic sequence for (20, A) code.

TABLE 12 i M_(i,0) M_(i,i) M_(i,2) M_(i,3) M_(i,4) M_(i,5) M_(i,6) M_(i,7) M_(i,8) M_(i,9) M_(i,10) M_(i,11) M_(i,12)  0 1 1 0 0 0 0 0 0 0 0 1 1 0  1 1 1 1 0 0 0 0 0 0 1 1 1 0  2 1 0 0 1 0 0 1 0 1 1 1 1 1  3 1 0 1 1 0 0 0 0 1 0 1 1 1  4 1 1 1 1 0 0 0 1 0 0 1 1 1  5 1 1 0 0 1 0 1 1 1 0 1 1 1  6 1 0 1 0 1 0 1 0 1 1 1 1 1  7 1 0 0 1 1 0 0 1 1 0 1 1 1  8 1 1 0 1 1 0 0 1 0 1 1 1 1  9 1 0 1 1 1 0 1 0 0 1 1 1 1 10 1 0 1 0 0 1 1 1 0 1 1 1 1 11 1 1 1 0 0 1 1 0 1 0 1 1 1 12 1 0 0 1 0 1 0 1 1 1 1 1 1 13 1 1 0 1 0 1 0 1 0 1 1 1 1 14 1 0 0 0 1 1 0 1 0 0 1 0 1 15 1 1 0 0 1 1 1 1 0 1 1 0 1 16 1 1 1 0 1 1 1 0 0 1 0 1 1 17 1 0 0 1 1 1 0 0 1 0 0 1 1 18 1 1 0 1 1 1 1 1 0 0 0 0 0 19 1 0 0 0 0 1 1 0 0 0 0 0 0

Channel coding bits b₀, b₁, b₂, b₃, . . . , b_(B−1) may be generated by [Equation 31].

$\begin{matrix} {b_{i} = {\sum\limits_{n = 0}^{A - 1}{\left( {a_{n} \cdot M_{i,n}} \right){mod}\; 2}}} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack \end{matrix}$

In [Equation 3], ‘i=0, 1, 2, . . . , B−1’ is met.

In case of wideband repots, a bandwidth of UCI (uplink control information) field for CQI/PMI can be represented as Tables 8 to 10 in the following.

[Table 13] shows UCI (Uplink Control Information) field for broadband report (single antenna port, transmit diversity) or open loop spatial multiplexing PDSCH CQI feedback.

TABLE 13 Field Bandwidth Wideband CQI 4

[Table 14] shows UL control information (UCI) field for CQI and PMI feedback in case of wideband reports (closed loop spatial multiplexing PDSCH transmission).

TABLE 14 Bandwidth 2 antenna ports 4 antenna ports Field rank = 1 rank = 2 rank = 1 Rank > 1 Wideband CQI 4 4 4 4 Spatial differential CQI 0 3 0 3 Precoding Matrix 2 1 4 4 Indication

[Table 15] shows UL control information (UCI) field for RI feedback in case of wideband reports.

TABLE 15 Bit widths 4 antenna ports Field 2 antenna ports Max. 2 layers Max. 4 layers Rank Indication 1 1 2

FIG. 12 is a diagram for PRB allocation. Referring to FIG. 20, PRB may be usable for PUCCH transmission in a slot n_(s).

2. Carrier Aggregation (CA) Environment

2.1 CA Overview

A 3GPP LTE system (conforming to Rel-8 or Rel-9) (hereinafter, referred to as an LTE system) uses Multi-Carrier Modulation (MCM) in which a single Component Carrier (CC) is divided into a plurality of bands. In contrast, a 3GPP LTE-A system (hereinafter, referred to an LTE-A system) may use CA by aggregating one or more CCs to support a broader system bandwidth than the LTE system. The term CA is interchangeably used with carrier combining, multi-CC environment, or multi-carrier environment.

In the present disclosure, multi-carrier means CA (or carrier combining). Herein, CA covers aggregation of contiguous carriers and aggregation of non-contiguous carriers. The number of aggregated CCs may be different for a DL and a UL. If the number of DL CCs is equal to the number of UL CCs, this is called symmetric aggregation. If the number of DL CCs is different from the number of UL CCs, this is called asymmetric aggregation. The term CA is interchangeable with carrier combining, bandwidth aggregation, spectrum aggregation, etc.

The LTE-A system aims to support a bandwidth of up to 100 MHz by aggregating two or more CCs, that is, by CA. To guarantee backward compatibility with a legacy IMT system, each of one or more carriers, which has a smaller bandwidth than a target bandwidth, may be limited to a bandwidth used in the legacy system.

For example, the legacy 3GPP LTE system supports bandwidths {1.4, 3, 5, 10, 15, and 20 MHz} and the 3GPP LTE-A system may support a broader bandwidth than 20 MHz using these LTE bandwidths. A CA system of the present disclosure may support CA by defining a new bandwidth irrespective of the bandwidths used in the legacy system.

There are two types of CA, intra-band CA and inter-band CA. Intra-band CA means that a plurality of DL CCs and/or UL CCs are successive or adjacent in frequency. In other words, the carrier frequencies of the DL CCs and/or UL CCs are positioned in the same band. On the other hand, an environment where CCs are far away from each other in frequency may be called inter-band CA. In other words, the carrier frequencies of a plurality of DL CCs and/or UL CCs are positioned in different bands. In this case, a UE may use a plurality of Radio Frequency (RF) ends to conduct communication in a CA environment.

The LTE-A system adopts the concept of cell to manage radio resources. The above-described CA environment may be referred to as a multi-cell environment. A cell is defined as a pair of DL and UL CCs, although the UL resources are not mandatory. Accordingly, a cell may be configured with DL resources alone or DL and UL resources.

For example, if one serving cell is configured for a specific UE, the UE may have one DL CC and one UL CC. If two or more serving cells are configured for the UE, the UE may have as many DL CCs as the number of the serving cells and as many UL CCs as or fewer UL CCs than the number of the serving cells, or vice versa. That is, if a plurality of serving cells are configured for the UE, a CA environment using more UL CCs than DL CCs may also be supported.

CA may be regarded as aggregation of two or more cells having different carrier frequencies (center frequencies). Herein, the term ‘cell’ should be distinguished from ‘cell’ as a geographical area covered by an eNB. Hereinafter, intra-band CA is referred to as intra-band multi-cell and inter-band CA is referred to as inter-band multi-cell.

In the LTE-A system, a Primacy Cell (PCell) and a Secondary Cell (SCell) are defined. A PCell and an SCell may be used as serving cells. For a UE in RRC_CONNECTED state, if CA is not configured for the UE or the UE does not support CA, a single serving cell including only a PCell exists for the UE. On the contrary, if the UE is in RRC_CONNECTED state and CA is configured for the UE, one or more serving cells may exist for the UE, including a PCell and one or more SCells.

Serving cells (PCell and SCell) may be configured by an RRC parameter. A physical-layer ID of a cell, PhysCellId is an integer value ranging from 0 to 503. A short ID of an SCell, SCellIndex is an integer value ranging from 1 to 7. A short ID of a serving cell (PCell or SCell), ServeCellIndex is an integer value ranging from 1 to 7. If ServeCellIndex is 0, this indicates a PCell and the values of ServeCellIndex for SCells are pre-assigned. That is, the smallest cell ID (or cell index) of ServeCellIndex indicates a PCell.

A PCell refers to a cell operating in a primary frequency (or a primary CC). A UE may use a PCell for initial connection establishment or connection reestablishment. The PCell may be a cell indicated during handover. In addition, the PCell is a cell responsible for control-related communication among serving cells configured in a CA environment. That is, PUCCH allocation and transmission for the UE may take place only in the PCell. In addition, the UE may use only the PCell in acquiring system information or changing a monitoring procedure. An Evolved Universal Terrestrial Radio Access Network (E-UTRAN) may change only a PCell for a handover procedure by a higher layer RRCConnectionReconfiguraiton message including mobilityControlInfo to a UE supporting CA.

An SCell may refer to a cell operating in a secondary frequency (or a secondary CC). Although only one PCell is allocated to a specific UE, one or more SCells may be allocated to the UE. An SCell may be configured after RRC connection establishment and may be used to provide additional radio resources. There is no PUCCH in cells other than a PCell, that is, in SCells among serving cells configured in the CA environment.

When the E-UTRAN adds an SCell to a UE supporting CA, the E-UTRAN may transmit all system information related to operations of related cells in RRC_CONNECTED state to the UE by dedicated signaling. Changing system information may be controlled by releasing and adding a related SCell. Herein, a higher layer RRCConnectionReconfiguration message may be used. The E-UTRAN may transmit a dedicated signal having a different parameter for each cell rather than it broadcasts in a related SCell.

After an initial security activation procedure starts, the E-UTRAN may configure a network including one or more SCells by adding the SCells to a PCell initially configured during a connection establishment procedure. In the CA environment, each of a PCell and an SCell may operate as a CC. Hereinbelow, a Primary CC (PCC) and a PCell may be used in the same meaning and a Secondary CC (SCC) and an SCell may be used in the same meaning in embodiments of the present disclosure.

FIG. 13 illustrates an example of CCs and CA in the LTE-A system, which are used in embodiments of the present disclosure.

FIG. 13(a) illustrates a single carrier structure in the LTE system. There are a DL CC and a UL CC and one CC may have a frequency range of 20 MHz.

FIG. 13(b) illustrates a CA structure in the LTE-A system. In the illustrated case of FIG. 13(b), three CCs each having 20 MHz are aggregated. While three DL CCs and three UL CCs are configured, the numbers of DL CCs and UL CCs are not limited. In CA, a UE may monitor three CCs simultaneously, receive a DL signal/DL data in the three CCs, and transmit a UL signal/UL data in the three CCs.

If a specific cell manages N DL CCs, the network may allocate M (M≤N) DL CCs to a UE. The UE may monitor only the M DL CCs and receive a DL signal in the M DL CCs. The network may prioritize L (L≤M≤N) DL CCs and allocate a main DL CC to the UE. In this case, the UE should monitor the L DL CCs. The same thing may apply to UL transmission.

The linkage between the carrier frequencies of DL resources (or DL CCs) and the carrier frequencies of UL resources (or UL CCs) may be indicated by a higher layer message such as an RRC message or by system information. For example, a set of DL resources and UL resources may be configured based on linkage indicated by System Information Block Type 2 (SIB2). Specifically, DL-UL linkage may refer to a mapping relationship between a DL CC carrying a PDCCH with a UL grant and a UL CC using the UL grant, or a mapping relationship between a DL CC (or a UL CC) carrying HARQ data and a UL CC (or a DL CC) carrying an HARQ ACK/NACK signal.

2.2 Cross Carrier Scheduling

Two scheduling schemes, self-scheduling and cross carrier scheduling are defined for a CA system, from the perspective of carriers or serving cells. Cross carrier scheduling may be called cross CC scheduling or cross cell scheduling.

In self-scheduling, a PDCCH (carrying a DL grant) and a PDSCH are transmitted in the same DL CC or a PUSCH is transmitted in a UL CC linked to a DL CC in which a PDCCH (carrying a UL grant) is received.

In cross carrier scheduling, a PDCCH (carrying a DL grant) and a PDSCH are transmitted in different DL CCs or a PUSCH is transmitted in a UL CC other than a UL CC linked to a DL CC in which a PDCCH (carrying a UL grant) is received.

Cross carrier scheduling may be activated or deactivated UE-specifically and indicated to each UE semi-statically by higher layer signaling (e.g. RRC signaling).

If cross carrier scheduling is activated, a Carrier Indicator Field (CIF) is required in a PDCCH to indicate a DL/UL CC in which a PDSCH/PUSCH indicated by the PDCCH is to be transmitted. For example, the PDCCH may allocate PDSCH resources or PUSCH resources to one of a plurality of CCs by the CIF. That is, when a PDCCH of a DL CC allocates PDSCH or PUSCH resources to one of aggregated DL/UL CCs, a CIF is set in the PDCCH. In this case, the DCI formats of LTE Release-8 may be extended according to the CIF. The CIF may be fixed to three bits and the position of the CIF may be fixed irrespective of a DCI format size. In addition, the LTE Release-8 PDCCH structure (the same coding and resource mapping based on the same CCEs) may be reused.

On the other hand, if a PDCCH transmitted in a DL CC allocates PDSCH resources of the same DL CC or allocates PUSCH resources in a single UL CC linked to the DL CC, a CIF is not set in the PDCCH. In this case, the LTE Release-8 PDCCH structure (the same coding and resource mapping based on the same CCEs) may be used.

If cross carrier scheduling is available, a UE needs to monitor a plurality of PDCCHs for DCI in the control region of a monitoring CC according to the transmission mode and/or bandwidth of each CC. Accordingly, an appropriate SS configuration and PDCCH monitoring are needed for the purpose.

In the CA system, a UE DL CC set is a set of DL CCs scheduled for a UE to receive a PDSCH, and a UE UL CC set is a set of UL CCs scheduled for a UE to transmit a PUSCH. A PDCCH monitoring set is a set of one or more DL CCs in which a PDCCH is monitored. The PDCCH monitoring set may be identical to the UE DL CC set or may be a subset of the UE DL CC set. The PDCCH monitoring set may include at least one of the DL CCs of the UE DL CC set. Or the PDCCH monitoring set may be defined irrespective of the UE DL CC set. DL CCs included in the PDCCH monitoring set may be configured to always enable self-scheduling for UL CCs linked to the DL CCs. The UE DL CC set, the UE UL CC set, and the PDCCH monitoring set may be configured UE-specifically, UE group-specifically, or cell-specifically.

If cross carrier scheduling is deactivated, this implies that the PDCCH monitoring set is always identical to the UE DL CC set. In this case, there is no need for signaling the PDCCH monitoring set. However, if cross carrier scheduling is activated, the PDCCH monitoring set may be defined within the UE DL CC set. That is, the eNB transmits a PDCCH only in the PDCCH monitoring set to schedule a PDSCH or PUSCH for the UE.

FIG. 14 illustrates a cross carrier-scheduled subframe structure in the LTE-A system, which is used in embodiments of the present disclosure.

Referring to FIG. 14, three DL CCs are aggregated for a DL subframe for LTE-A UEs. DL CC ‘A’ is configured as a PDCCH monitoring DL CC. If a CIF is not used, each DL CC may deliver a PDCCH that schedules a PDSCH in the same DL CC without a CIF. On the other hand, if the CIF is used by higher layer signaling, only DL CC ‘A’ may carry a PDCCH that schedules a PDSCH in the same DL CC ‘A’ or another CC. Herein, no PDCCH is transmitted in DL CC ‘B’ and DL CC ‘C’ that are not configured as PDCCH monitoring DL CCs.

FIG. 15 is conceptual diagram illustrating a construction of serving cells according to cross-carrier scheduling.

Referring to FIG. 15, an eNB (or BS) and/or UEs for use in a radio access system supporting carrier aggregation (CA) may include one or more serving cells. In FIG. 8, the eNB can support a total of four serving cells (cells A, B, C and D). It is assumed that UE A may include Cells (A, B, C), UE B may include Cells (B, C, D), and UE C may include Cell B. In this case, at least one of cells of each UE may be composed of P Cell. In this case, P Cell is always activated, and SCell may be activated or deactivated by the eNB and/or UE.

The cells shown in FIG. 15 may be configured per UE. The above-mentioned cells selected from among cells of the eNB, cell addition may be applied to carrier aggregation (CA) on the basis of a measurement report message received from the UE. The configured cell may reserve resources for ACK/NACK message transmission in association with PDSCH signal transmission. The activated cell is configured to actually transmit a PDSCH signal and/or a PUSCH signal from among the configured cells, and is configured to transmit CSI reporting and Sounding Reference Signal (SRS) transmission. The deactivated cell is configured not to transmit/receive PDSCH/PUSCH signals by an eNB command or a timer operation, and CRS reporting and SRS transmission are interrupted.

2.4. Channel State Information (CSI) Feedback on PUCCH

First of all, in the 3GPP LTE system, when a DL reception entity (e.g., UE) is connected to a DL transmission entity (e.g., BS), the DL reception entity performs measurement on a Reference Signal Received Power (RSRP) of a reference signal transmitted in DL, a quality of a reference signal (RSRQ: Reference Signal Received Quality) and the like at a random time and is then able to make a periodic or even-triggered report of a corresponding measurement result to the BS.

Each UE reports a DL channel information in accordance with a DL channel status via uplink. A base station is then able to determine time/frequency resources, MCS (modulation and coding scheme) and the like appropriate for a data transmission to each UE using the DL channel information received from the each UE.

Such Channel State Information (CSI) may include Channel Quality Indicator (CQI), Precoding Matrix Indicator (PMI), Precoder Type Indication (PTI) and/or Rank Indication (RI). In particular, the CSI may be transmitted entirely or partially depending on a transmission mode of each UE. CQI is determined based on a received signal quality of a UE, which may be generally determined on the basis of a measurement of a DL reference signal. In doing so, a CQI value actually delivered to a base station may correspond to an MCS capable of providing maximum performance by maintaining a Block Error Rate (BLER) under 10% in the received signal quality measured by a UE.

This channel information reporting may be classified into a periodic report transmitted periodically and an aperiodic report transmitted in response to a request made by a BS.

In case of the aperiodic report, it is set for each UE by a 1-bit request bit (CQI request bit) contained in UL scheduling information downloaded to a UE by a BS. Having received this information, each UE is then able to deliver channel information to the BS via a Physical Uplink Shared Channel (PUSCH) in consideration of its transmission mode. And, it may set RI and CQI/PMI not to be transmitted on the same PUSCH.

In case of the periodic report, a period for transmitting channel information via an upper layer signal, an offset in the corresponding period and the like are signaled to each UE by subframe unit and channel information in consideration of a transmission mode of each UE may be delivered to a BS via a Physical Uplink Control Channel (PUCCH) in accordance with a determined period. In case that data transmitted in uplink simultaneously exists in a subframe in which channel information is transmitted by a determined period, the corresponding channel information may be transmitted together with the data not on the PUCCH but on a Physical Uplink Shared Channel (PUSCH). In case of the periodic report via PUCCH, bits (e.g., 11 bits) limited further than those of the PUSCH may be used. RI and CQI/PMI may be transmitted on the same PUSCH.

In case that contention occurs between the periodic report and the aperiodic report in the same subframe, only the aperiodic report can be performed.

In calculating Wideband CQI/PMI, a most recently transmitted RI may be usable. RI in a PUCCH CSI report mode is independent from RI in a PUSCH CSI report mode. The RI in the PUSCH CSI report mode is valid for CQI/PMI in the corresponding PUSCH CSI report mode only.

Table 16 is provided to describe CSI feedback type transmitted on PUCCH and PUCCH CSI report mode.

TABLE 16 PMI Feedback Type No PMI (OL, TD, single-antenna) Single PMI (CL) CQI Wideband Mode 1-0 Mode 1-1 Feedback RI (only for Open-Loop SM) RI Type One Wideband CQI (4 bit) Wideband CQI (4 bit) when RI > 1, CQI of first codeword Wideband spatial CQI (3 bit) for RI > 1 Wideband PMI (4 bit) UE Mode 2-0 Mode 2-1 Selected RI (only for Open-Loop SM) RI Wideband CQI (4 bit) Wideband CQI (4 bit) Best-1 CQI (4 bit) in each BP Wideband spatial CQI (3 bit) for RI > 1 Best-1 indicator(L-bit label) Wideband PMI (4 bit) when RI > 1, CQI of first codeword Best-1 CQI (4 bit) 1 in each BP Best-1 spatial CQI (3 bit) for RI > 1 Best-1 indicator (L-bit label)

Referring to [Table 16], in the periodic report of channel information, there are 4 kinds of reporting modes (mode 1-0, mode 1-2, mode 2-0 and mode 2-1) in accordance with CQI and PMI feedback types.

CQI can be classified into WideBand (WB) CQI and SubBand (SB) CQI in accordance with CQI feedback type and PMI can be classified into No PMI or Single PMI in accordance with a presence or non-presence of PMI transmission. In Table 11, No PMI corresponds to a case of Open-Loop (OL), Transmit Diversity (TD) and single-antenna, while Single PMI corresponds to a case of Closed-Loop (CL).

The mode 1-0 corresponds to a case that WB CQI is transmitted in the absence of PMI transmission. In this case, RI is transmitted only in case of OL Spatial Multiplexing (SM) and one WB CQI represented as 4 bits can be transmitted. If RI is greater than 1, CQI for a 1^(st) codeword can be transmitted.

Mode 1-1 corresponds to a case that a single PMI and WB CQI are transmitted. In this case, 4-bit WB CQI and 4-bit WB PMI can be transmitted together with RI transmission. Additionally, if RI is greater than 1, 3-bit WB (wideband) spatial differential CQI can be transmitted. In 2-codeword transmission, the WB spatial differential CQI may indicate a difference value between a WB CQI index for codeword 1 and a WB CQI index for codeword 2. The difference value in-between may have a value selected from a set {−4, −3, −2, −1, 0, 1, 2, 3} and can be represented as 3 bits.

The mode 2-0 corresponds to a case that CQI on a UE-selected band is transmitted in the absence of PMI transmission. In this case, RI is transmitted only in case of open-loop spatial multiplexing (SM) and a WB CQI represented as 4 bits may be transmitted. A best CQI (best-1) is transmitted on each bandwidth part (BP) and the best-1 CQI may be represented as 4 bits. And, an L-bit indicator indicating the best-1 may be transmitted together. If the RI is greater than 1, a CQI for a 1^(st) codeword can be transmitted.

And, Mode 2-1 corresponds to a case that a single PMI and a CQI on a UE-selected band are transmitted. In this case, together with RI transmission, 4-bit WB CQI, 3-bit WB spiral differential CQI and 4-bit WB PMI can be transmitted. Additionally, 4-bit best-1 CQI is transmitted on each Bandwidth Part (BP) and L-bit best-1 indicator can be transmitted together. Additionally, if RI is greater than 1, 3-bit best-1 spatial differential CQI can be transmitted. In 2-codeword transmission, it may indicate a difference value between a best-1 CQI index of codeword 1 and a best-1 CQI index of codeword 2.

For the transmission modes, periodic PUCCH CSI report modes are supported as follows.

1) Transmission mode 1: Modes 1-0 and 2-0

2) Transmission mode 2: Modes 1-0 and 2-0

3) Transmission mode 3: Modes 1-0 and 2-0

4) Transmission mode 4: Modes 1-1 and 2-1

5) Transmission mode 5: Modes 1-1 and 2-1

6) Transmission mode 6: Modes 1-1 and 2-1

7) Transmission mode 7: Modes 1-0 and 2-0

8) Transmission mode 8: Modes 1-1 and 2-1 if a UE is set to make a PMI/RI reporting, or Modes 1-0 and 2-0 if a UE is set not to make a PMI/RI reporting

9) Transmission mode 9: Modes 1-1 and 2-1 if a UE is set to make a PMI/RI reporting and the number of CSI-RS ports is greater than 1, or Modes 1-0 and 2-0 if a UE is set not to make a PMI/RI reporting and the number of CSI-RS port(s) is equal to 1.

The periodic PUCCH CSIU reporting mode in each serving cell is set by upper layer signaling. And, Mode 1-1 is set to either submode 1 or submode 2 by an upper layer signaling using a parameter ‘PUCCH_format1-1_CSI_reporting_mode’.

A CQI reporting in a specific subframe of a specific serving cell in a UE-selected SB CQI means a measurement of at least one channel state of a bandwidth part (BP) corresponding to a portion of a bandwidth of a serving cell. An index is given to the bandwidth part in a frequency increasing order starting with a lowest frequency without an increment of a bandwidth.

2.4 Method for Transmitting ACK/NACK on PUCCH

2.4.1 ACK/NACK Transmission in LTE System

Under the situation that a UE simultaneously transmits a plurality of ACKs/NACKs corresponding to multiple data units received from an eNB, in order to maintain the single-carrier property of ACK/NACK signals and reduce the total ACK/NACK transmission power, ACK/NACK multiplexing method based on PUCCH resource selection can be considered. With ACK/NACK multiplexing, contents of the ACK/NACK signals for multiple data units are identified by the combination of the PUCCH resource used in actual ACK/NACK transmission and the one of QPSK modulation symbols. For example, if it is assumed that one PUCCH resource carries 4 bits and 4 data units can be transmitted in maximum (at this time, assume that HARQ operation for each data unit can be managed by single ACK/NACK bit), the Transmission (Tx) node can identify the ACK/NACK result based on the transmission position of the PUCCH signal and the bits of the ACK/NACK signal as shown in [Table 17] below.

TABLE 17 HARQ-ACK(0), HARQ-ACK(1), b(0), HARQ-ACK(2), HARQ-ACK(3) n_(PUCCH) ⁽¹⁾ b(1) ACK, ACK, ACK, ACK n_(PUCCH, 1) ⁽¹⁾ 1, 1 ACK, ACK, ACK, NACK/DTX n_(PUCCH, 1) ⁽¹⁾ 1, 0 NACK/DTX, NACK/DTX, NACK, DTX n_(PUCCH, 2) ⁽¹⁾ 1, 1 ACK, ACK, NACK/DTX, ACK n_(PUCCH, 1) ⁽¹⁾ 1, 0 NACK, DTX, DTX, DTX n_(PUCCH, 0) ⁽¹⁾ 1, 0 ACK, ACK, NACK/DTX, NACK/DTX n_(PUCCH, 1) ⁽¹⁾ 1, 0 ACK, NACK/DTX, ACK, ACK n_(PUCCH, 3) ⁽¹⁾ 0, 1 NACK/DTX, NACK/DTX, NACK/DTX, NACK n_(PUCCH, 3) ⁽¹⁾ 1, 1 ACK, NACK/DTX, ACK, NACK/DTX n_(PUCCH, 2) ⁽¹⁾ 0, 1 ACK, NACK/DTX, NACK/DTX, ACK n_(PUCCH, 0) ⁽¹⁾ 0, 1 ACK, NACK/DTX, NACK/DTX, NACK/DTX n_(PUCCH, 0) ⁽¹⁾ 1, 1 NACK/DTX, ACK, ACK, ACK n_(PUCCH, 3) ⁽¹⁾ 0, 1 NACK/DTX, NACK, DTX, DTX n_(PUCCH, 1) ⁽¹⁾ 0, 0 NACK/DTX, ACK, ACK, NACK/DTX n_(PUCCH, 2) ⁽¹⁾ 1, 0 NACK/DTX, ACK, NACK/DTX, ACK n_(PUCCH, 3) ⁽¹⁾ 1, 0 NACK/DTX, ACK, NACK/DTX, NACK/DTX n_(PUCCH, 1) ⁽¹⁾ 0, 1 NACK/DTX, NACK/DTX, ACK, ACK n_(PUCCH, 3) ⁽¹⁾ 0, 1 NACK/DTX, NACK/DTX, ACK, NACK/DTX n_(PUCCH, 2) ⁽¹⁾ 0, 0 NACK/DTX, NACK/DTX, NACK/DTX, ACK n_(PUCCH, 3) ⁽¹⁾ 0, 0 DTX, DTX, DTX, DTX N/A N/A

In [Table 17], HARQ-ACK(i) indicates the ACK/NACK result for the data unit i. For example, if a maximum of 4 data units is transmitted, i=0, 1, 2, 3. In Table 17, DTX means that there is no data unit transmitted for corresponding HARQ-ACK(i) or the Reception (Rx) node doesn't detect the existence of the data unit corresponding to HARQ-ACK(i).

In addition, n_(PUCCH,X) ⁽¹⁾ indicates the PUCCH resource which should be used in actual ACK/NACK transmission, if there are 4 PUCCH resources, a maximum of four PUCCH resources n_(PUCCH,0) ⁽¹⁾, n_(PUCCH,1) ⁽¹⁾, n_(PUCCH,2) ⁽¹⁾, and n_(PUCCH,3) ⁽¹⁾ may be allocated to the UE.

In addition, b(0), b(1) indicates two bits carried by the selected PUCCH resource. Modulation symbols which are transmitted through PUCCH resource are decided according to the bits. For example, if the RX node receives and decodes 4 data units successfully, the RX node should transmit two bits, (1, 1), using PUCCH resource n_(PUCCH,1) ⁽¹⁾. For another example, if the RX node receives 4 data units and fails in decoding the first and the third data units (corresponding to HARQ-ACK(0) and HARQ-ACK(2)), the RX node should transmit (1, 0) using n_(PUCCH,3) ⁽¹⁾.

By linking the actual ACK/NACK contents with the combination of PUCCH resource selection and the actual bit contents in the transmitted PUCCH resource in this way, ACK/NACK transmission using a single PUCCH resource for multiple data units is possible.

In ACK/NACK multiplexing method (see Table 17), basically, NACK and DTX are coupled as NACK/DTX if at least one ACK exists for all data units. This is because combinations of PUCCH resources and QPSK symbols are insufficient to cover all ACK, NACK and DTX hypotheses. On the other hand, for the case that no ACK exists for all data units (in other words, NACK or DTX only exists for all data units), single NACK decoupled with DTX is defined one as HARQ-ACK(i). In this case, PUCCH resource linked to the data unit corresponding to single NACK can be also reserved to transmit the signal of multiple ACKs/NACKs.

2.4.2 ACK/NACK Transmission in LTE-A System

In an LTE-A system (e.g., Rel-10, 11, 12, etc.), transmission of a plurality of ACK/NACK signals for a plurality of PDSCH signals, which is transmitted via a plurality of DL CCs, via a specific UL CC is considered. Unlike ACK/NACK transmission using PUCCH format 1a/1b of an LTE system, a plurality of ACK/NACK signals may be subjected to channel coding (e.g., Reed-Muller coding, Tail-biting convolutional coding, etc.) and then a plurality of ACK/NACK information/signals may be transmitted using PUCCH format 2 or a new PUCCH format (e.g., an E-PUCCH format) modified based on block spreading.

FIG. 16 shows an example of a new PUCCH format based on block spreading.

A block spreading scheme refers to a method for performing modulation using an SC-FDMA scheme unlike PUCCH format series 1 or 2 in an LTE system. The block spreading scheme refers to a scheme for time-domain spreading and transmitting a symbol sequence using an Orthogonal Cover Code (OCC) as shown in FIG. 16. That is, the symbol sequence is spread using the OCC to multiplex control signals of several UEs in the same RB.

In the above-described PUCCH format 2, one symbol sequence is transmitted over the time domain and UE multiplexing is performed using Cyclic Shift (CCS) of a CAZAC sequence. However, in the new PUCCH format based on block spreading, one symbol sequence is transmitted over the frequency domain and UE multiplexing is performed using time-domain spreading based on an OCC.

For example, as shown in FIG. 16, one symbol sequence may be generated as five SC-FDMA symbols by an OCC of length-5 (that is, SF=5). Although a total of 2 RS symbols is used during one slot in FIG. 16, various methods using three RS symbols and using an OCC of SF=4 may be used. At this time, the RS symbols may be generated from a CAZAC sequence having specific cyclic shift and may be transmitted in the form in which a specific OCC is applied (multiplied by) to a plurality of RS symbols of the time domain.

In the embodiments of the present invention, for convenience of description, a multi-ACK/NACK transmission scheme based on channel coding using PUCCH format 2 or a new PUCCH format (e.g., an E-PUCCH format) is defined as a “multi-bit ACK/NACK coding transmission method”.

The multi-bit ACK/NACK coding method refers to a method for transmitting ACK/NACK code blocks generated by channel-coding ACK/NACK or DTX information (meaning that the PDCCH is not received/detected) for PDSCH signals transmitted on a plurality of DL CCs.

For example, when the UE operates on a certain DL CC in an SU-MIMO mode and receives two CodeWords (CW), the UE may have a maximum of five feedback states including a total of four feedback states of each CW, such as ACK/ACK, ACK/NACK, NACK/ACK and NACK/NACK, and DTX. When the UE receives a single CW, the UE may have a maximum of three states including ACK, NACK and/or DTX. When NACK and DTX are equally processed, the UE may have a total of two states such as ACK and NACK/DTX.

Accordingly, when the UE aggregates a maximum of five DL CCs and the UE operates on all DL CCs in an SU-MIMO mode, the UE may have a maximum of 55 transmittable feedback states. At this time, the size of ACK/NACK payload representing the 55 feedback states may be a total of 12 bits. If DTX and NACK are equally processed, the number of feedback states becomes 45 and the size of the ACK/NACK payload representing the feedback states is a total of 10 bits.

In an ACK/NACK multiplexing (that is, ACK/NACK selection) method applied to an LTE TDD system, fundamentally, an implicit ACK/NACK selection method in which an implicit PUCCH resource corresponding to a PDCCH scheduling each PDSCH (that is, linked to a smallest CCE index) is used for ACK/NACK transmission in order to secure a PUCCH resource of each UE.

In an LTE-A FDD system, transmission of a plurality of ACK/NACK signals for a plurality of PDSCH signals transmitted via a plurality of DL CCs via one UE-specific UL CC is considered. “ACK/NACK selection” methods using an implicit PUCCH resource linked to a PDCCH scheduling some or all DL CCs (that is, linked to a smallest CCE index nCCE or linked to nCCE and nCCE+1) or a combination of an implicit PUCCH and an explicit PUCCH resource pre-allocated to each UE via RRC signaling are considered.

Even in an LTE-A TDD system, aggregation of a plurality of CCs is considered. For example, when a plurality of CCs is aggregated, UE transmitting a plurality of ACK/NACK information/signals for a plurality of PDSCH signals transmitted via a plurality of DL subframes and a plurality of CCs via a specific CC (that is, A/N CC) in UL subframes corresponding to the plurality of DL subframes in which the PDSCH signals are transmitted is considered.

At this time, unlike LTE-A FDD, a method (that is, full ACK/NACK) for transmitting a plurality of ACK/NACK signals corresponding to a maximum number of CWs, which may be transmitted via all CCs allocated to the UE, for a plurality of DL subframes may be considered or a method (that is, bundled ACK/NACK) for applying ACK/NACK bundling to a CW, CC and/or a subframe region, reducing the number of transmitted ACKs/NACKs and performing transmission may be considered.

At this time, CW bundling means that ACK/NACK bundling for CW per CC is applied to each DL subframe and CC bundling means that ACK/NACK bundling for all or some CCs is applied to each DL subframe. In addition, subframe bundling means that ACK/NACK bundling for all or some DL subframes is applied to each CC.

As the subframe bundling method, an ACK counter method indicating a total number of ACKs (or the number of some ACKs) per CC for all PDSCH signals or DL grant PDCCHs received on each DL CC may be considered. At this time, the multi-bit ACK/NACK coding scheme or the ACK/NACK transmission scheme based on the ACK/NACK selection method may be configurably applied according to the size of the ACK/NACK payload per UE, that is, the size of the ACK/NACK payload for transmission of full or bundled ACK/NACK configured per UE.

2.5 Procedure for Transmitting and Receiving PUCCH

In a mobile communication system, one eNB transmits and receives data to and from a plurality of UEs via a wireless channel environment in one cell/sector. In a system operating using multiple carriers or the like, the eNB receives packet traffic from a wired Internet network and transmits the received packet traffic to each UE using a predetermined communication scheme. At this time, downlink scheduling is how the eNB determines when data is transmitted to which UE using which frequency domain. In addition, the eNB receives and demodulates data from the UE using a predetermined communication scheme and transmits packet traffic over a wired Internet network. Uplink scheduling is how the eNB determines when to enable which UE to transmit uplink data using which frequency domain. In general, a UE having a good channel state may transmit and receive data using more time and frequency resources.

In a system operating using multiple carriers or the like, resources may be roughly divided into a time domain and a frequency domain. The resources may be defined as resource blocks, which includes N subcarriers and M subframes or predetermined time units. At this time, N and M may be 1. FIG. 17 is a diagram showing an example of configuring a resource block in time-frequency units.

In FIG. 17, one rectangle means one resource block and one resource block has several subcarriers on one axis and has a predetermined time unit (e.g., slots or subframes) on the other axis.

In downlink, an eNB schedules one or more resource blocks to a UE selected according to a determined scheduling rule and transmits data using resource bocks allocated to the UE. In uplink, the eNB schedules one or more resource blocks to a UE selected according to a predetermined scheduling rule and a UE transmits data in uplink using the allocated resources.

An error control method performed when a (sub)frame, in which data is transmitted and received, is lost or damaged after transmitting and receiving data after scheduling includes an Automatic Repeat reQuest (ARQ) method and a Hybrid ARQ (HARQ) method.

In the ARQ method, fundamentally, a transmitter waits for an acknowledgement (ACK) message after transmitting one (sub)frame and a receiver sends the ACK only upon receiving the sub(frame). When an error occurs in the (sub)frame, a negative ACK (NAK) message is sent and information on a reception frame, in which an error occurs, is removed from a receiver buffer. The transmitter transmits a subsequent (sub)frame upon receiving the ACK message but retransmits the (sub)frame upon receiving the NAK message. Unlike the ARQ method, in the HARQ method, when the received frame cannot be demodulated, the receiver transmits the NAK message to the transmitter, but the received frame is stored in a buffer during a predetermined time and is combined with a retransmitted frame, thereby increasing a reception success rate.

Recently, a HARQ method more efficient than the ARQ method is widely used. The HARQ method may be divided into various methods. For example, the HARQ method may be divided into a synchronous HARQ method and an asynchronous HARQ method according to retransmission timing and into a channel-adaptive HARQ method and a channel-non-adaptive HARQ method depending on whether the amount of resources used for retransmission is influenced by a channel state.

The synchronous HARQ method refers to a method of performing subsequent retransmission at timing determined by a system when initial transmission fails. For example, if it is assumed that retransmission is performed every four time units after initial transmission fails, retransmission timing is predetermined between the eNB and the UE and is not signaled. However, when the data transmission side receives a NAK message, the frame is retransmitted every four time units until an ACK message is received.

Meanwhile, the asynchronous HARQ method may be performed by newly scheduling retransmission timing or via additional signaling. The retransmission timing of the previously failed frame may be changed by several factors such as channel state.

The channel-non-adaptive HARQ method refers to a method of using scheduling information (e.g., the modulation method of the frame, the number of used resource blocks, Adaptive Modulation and Coding (AMC), etc.), which is set upon initial transmission, upon retransmission. In contrast, the channel-adaptive HARQ method refers to a method of changing such scheduling information according to the channel state.

For example, in the channel-non-adaptive HARQ method, a transmission side transmits data using six resource blocks upon initial transmission and retransmits data using six resource blocks upon retransmission. In contrast, in the channel-adaptive HARQ method, initial transmission is performed using six resource blocks and retransmission is performed using greater or less than six resource blocks according to the channel state.

Although there are four HARQ methods, the asynchronous and channel-adaptive HARQ method and the synchronous and channel-non-adaptive HARQ method are mainly used. The asynchronous and channel-adaptive HARQ method may maximize retransmission efficiency by adaptively changing the retransmission timing and the amount of used resources according to the channel state but may increase overhead. Accordingly, the asynchronous and channel-adaptive HARQ method is not generally considered for uplink. In contrast, the synchronous and channel-non-adaptive HARQ method may not cause overhead because retransmission timing and resource allocation are predetermined in the system, but has very low retransmission efficiency in a considerably changed channel state.

To this end, in the current 3GPP LTE/LTE-A system, the asynchronous HARQ method is used in downlink and the synchronous HARQ method is used in uplink.

FIG. 18 is a diagram showing an example of a resource allocation and retransmission method of an asynchronous HARQ method.

When an eNB transmits scheduling information in downlink, receives ACK/NAK information from a UE, and transmits next data, time delay occurs as shown in FIG. 19. This is channel propagation delay and delay occurring due to a time required for data decoding and data encoding.

A method of performing transmission using an independent HARQ process for data transmission without a gap during a delay period is being used. For example, if a shortest period from first data transmission to next data transmission is 7 subframes, data may be transmitted without a gap by setting 7 independent HARQ processes. In an LTE/LTE-A system, a maximum of eight HARQ processes may be allocated to one UE in non-MIMO.

2.6 CA Environment-Based CoMP Operation

Hereinafter, a cooperation multi-point (CoMP) transmission operation applicable to the embodiments of the present disclosure will be described.

In the LTE-A system, CoMP transmission may be implemented using a carrier aggregation (CA) function in the LTE. FIG. 19 is a conceptual view illustrating a CoMP system operating based on a CA environment.

In FIG. 19, it is assumed that a carrier operated as a PCell and a carrier operated as an SCell may use the same frequency band on a frequency axis and are allocated to two eNBs geographically spaced apart from each other. At this time, a serving eNB of UE1 may be allocated to the PCell, and a neighboring cell causing much interference may be allocated to the SCell. That is, the eNB of the PCell and the eNB of the SCell may perform various DL/UL CoMP operations such as joint transmission (JT), CS/CB and dynamic cell selection for one UE.

FIG. 19 illustrates an example that cells managed by two eNBs are aggregated as PCell and SCell with respect to one UE (e.g., UE1). However, as another example, three or more cells may be aggregated. For example, some cells of three or more cells may be configured to perform CoMP operation for one UE in the same frequency band, and the other cells may be configured to perform simple CA operation in different frequency bands. At this time, the PCell does not always need to participate in CoMP operation.

2.7 Reference Signal (RS)

Now, a description will be given of RSs which may be used in embodiments of the present disclosure.

FIG. 20 illustrates an example of a subframe to which UE-RSs are allocated, which may be used in embodiments of the present disclosure.

Referring to FIG. 20, the subframe illustrates REs occupied by UE-RSs among REs in one RB of a normal DL subframe having a normal CP.

UE-RSs are transmitted on antenna port(s) p=5, p=7, p=8 or p=7, 8, . . . , υ+6 for PDSCH transmission, where i) is the number of layers used for the PDSCH transmission. UE-RSs are present and are a valid reference for PDSCH demodulation only if the PDSCH transmission is associated with the corresponding antenna port. UE-RSs are transmitted only on RBs to which the corresponding PDSCH is mapped.

The UE-RSs are configured to be transmitted only on RB(s) to which a PDSCH is mapped in a subframe in which the PDSCH is scheduled unlike CRSs configured to be transmitted in every subframe irrespective of whether the PDSCH is present. Accordingly, overhead of the RS may decrease relative to overhead of the CRS.

In the 3GPP LTE-A system, the UE-RSs are defined in a PRB pair. Referring to FIG. 19, in a PRB having frequency-domain index nPRB assigned for PDSCH transmission with respect to p=7, p=8, or p=7, 8, . . . , υ+6, a part of UE-RS sequence r(m) is mapped to complex-valued modulation symbols.

UE-RSs are transmitted through antenna port(s) corresponding respectively to layer(s) of a PDSCH. That is, the number of UE-RS ports is proportional to a transmission rank of the PDSCH. Meanwhile, if the number of layers is 1 or 2, 12 REs per RB pair are used for UE-RS transmission and, if the number of layers is greater than 2, 24 REs per RB pair are used for UE-RS transmission. In addition, locations of REs occupied by UE-RSs (i.e. locations of UE-RS REs) in a RB pair are the same with respect to a UE-RS port regardless of a UE or a cell.

As a result, the number of DM-RS REs in an RB to which a PDSCH for a specific UE in a specific subframe is mapped is the same per UE-RS ports. Notably, in RBs to which the PDSCH for different UEs in the same subframe is allocated, the number of DM-RS REs included in the RBs may differ according to the number of transmitted layers.

The UE-RS can be used as the DM-RS in the embodiments of the present disclosure.

2.8 Enhanced PDCCH (EPDCCH)

In the 3GPP LTE/LTE-A system, Cross-Carrier Scheduling (CCS) in an aggregation status for a plurality of component carriers (CC: component carrier=(serving) cell) will be defined. One scheduled CC may previously be configured to be DL/UL scheduled from another one scheduling CC (that is, to receive DL/UL grant PDCCH for a corresponding scheduled CC). At this time, the scheduling CC may basically perform DL/UL scheduling for itself. In other words, a Search Space (SS) for a PDCCH for scheduling scheduling/scheduled CCs which are in the CCS relation may exist in a control channel region of all the scheduling CCs.

Meanwhile, in the LTE system, FDD DL carrier or TDD DL subframes are configured to use first n (n<=4) OFDM symbols of each subframe for transmission of physical channels for transmission of various kinds of control information, wherein examples of the physical channels include a PDCCH, a PHICH, and a PCFICH. At this time, the number of OFDM symbols used for control channel transmission at each subframe may be delivered to the UE dynamically through a physical channel such as PCFICH or semi-statically through RRC signaling.

Meanwhile, in the LTE/LTE-A system, since a PDCCH which is a physical channel for DL/UL scheduling and transmitting various kinds of control information has a limitation that it is transmitted through limited OFDM symbols, enhanced PDCCH (i.e., E-PDCCH) multiplexed with a PDSCH more freely in a way of FDM/TDM may be introduced instead of a control channel such as PDCCH, which is transmitted through OFDM symbol and separated from PDSCH. FIG. 21 illustrates an example that legacy PDCCH, PDSCH and E-PDCCH, which are used in an LTE/LTE-A system, are multiplexed.

3. LTE-U System

3.1 LTE-U System Configuration

Hereinafter, methods for transmitting and receiving data in a CA environment of an LTE-A band corresponding to a licensed band and an unlicensed band will be described. In the embodiments of the present disclosure, an LTE-U system means an LTE system that supports such a CA status of a licensed band and an unlicensed band. A WiFi band or Bluetooth (BT) band may be used as the unlicensed band. LTE-A system operating in an unlicensed band is referred to as LAA (licensed assisted access). Or, the LAA may correspond to a scheme of performing data transmission/reception in an unlicensed band in a manner of being combined with a licensed band.

FIG. 22 illustrates an example of a CA environment supported in an LTE-U system.

Hereinafter, for convenience of description, it is assumed that a UE is configured to perform wireless communication in each of a licensed band and an unlicensed band by using two CCs. The methods which will be described hereinafter may be applied to even a case where three or more CCs are configured for a UE.

In the embodiments of the present disclosure, it is assumed that a carrier of the licensed band may be a primary CC (PCC or PCell), and a carrier of the unlicensed band may be a secondary CC (SCC or SCell). However, the embodiments of the present disclosure may be applied to even a case where a plurality of licensed bands and a plurality of unlicensed bands are used in a carrier aggregation method. Also, the methods suggested in the present disclosure may be applied to even a 3GPP LTE system and another system.

In FIG. 22, one eNB supports both a licensed band and an unlicensed band. That is, the UE may transmit and receive control information and data through the PCC which is a licensed band, and may also transmit and receive control information and data through the SCC which is an unlicensed band. However, the status shown in FIG. 22 is only example, and the embodiments of the present disclosure may be applied to even a CA environment that one UE accesses a plurality of eNBs.

For example, the UE may configure a macro eNB (M-eNB) and a PCell, and may configure a small eNB (S-eNB) and an SCell. At this time, the macro eNB and the small eNB may be connected with each other through a backhaul network.

In the embodiments of the present disclosure, the unlicensed band may be operated in a contention-based random access method. At this time, the eNB that supports the unlicensed band may perform a Carrier Sensing (CS) procedure prior to data transmission and reception. The CS procedure determines whether a corresponding band is reserved by another entity.

For example, the eNB of the SCell checks whether a current channel is busy or idle. If it is determined that the corresponding band is idle state, the eNB may transmit a scheduling grant to the UE to allocate a resource through (E)PDCCH of the PCell in case of a cross carrier scheduling mode and through PDCCH of the SCell in case of a self-scheduling mode, and may try data transmission and reception.

At this time, the eNB may configure a TxOP including N consecutive subframes. In this case, a value of N and a use of the N subframes may previously be notified from the eNB to the UE through higher layer signaling through the PCell or through a physical control channel or physical data channel.

3.2 Carrier Sensing (CS) Procedure

In embodiments of the present disclosure, a CS procedure may be called a Clear Channel Assessment (CCA) procedure. In the CCA procedure, it may be determined whether a channel is busy or idle based on a predetermined CCA threshold or a CCA threshold configured by higher-layer signaling. For example, if energy higher than the CCA threshold is detected in an unlicensed band, SCell, it may be determined that the channel is busy or idle. If the channel is determined to be idle, an eNB may start signal transmission in the SCell. This procedure may be referred to as LBT.

FIG. 23 is a view illustrating an exemplary Frame Based Equipment (FBE) operation as one of LBT operations.

The European Telecommunication Standards Institute (ETSI) regulation (EN 301 893 V1.7.1) defines two LBT operations, Frame Based Equipment (FBE) and Load Based Equipment (LBE). In FBE, one fixed frame is comprised of a channel occupancy time (e.g., 1 to 10 ms) being a time period during which a communication node succeeding in channel access may continue transmission, and an idle period being at least 5% of the channel occupancy time, and CCA is defined as an operation for monitoring a channel during a CCA slot (at least 20 μs) at the end of the idle period.

A communication node periodically performs CCA on a per-fixed frame basis. If the channel is unoccupied, the communication node transmits data during the channel occupancy time. On the contrary, if the channel is occupied, the communication node defers the transmission and waits until the CCA slot of the next period.

FIG. 24 is a block diagram illustrating the FBE operation.

Referring to FIG. 24, a communication node (i.e., eNB) managing an SCell performs CCA during a CCA slot. If the channel is idle, the communication node performs data transmission (Tx). If the channel is busy, the communication node waits for a time period calculated by subtracting the CCA slot from a fixed frame period, and then resumes CCA.

The communication node transmits data during the channel occupancy time. Upon completion of the data transmission, the communication node waits for a time period calculated by subtracting the CCA slot from the idle period, and then resumes CCA. If the channel is idle but the communication node has no transmission data, the communication node waits for the time period calculated by subtracting the CCA slot from the fixed frame period, and then resumes CCA.

FIG. 25 is a view illustrating an exemplary LBE operation as one of the LBT operations.

Referring to FIG. 25(a), in LBE, the communication node first sets q (q ∈{4, 5, . . . , 32}) and then performs CCA during one CCA slot.

FIG. 25(b) is a block diagram illustrating the LBE operation. The LBE operation will be described with reference to FIG. 25(b).

The communication node may perform CCA during a CCA slot. If the channel is unoccupied in a first CCA slot, the communication node may transmit data by securing a time period of up to (13/32)q ms.

On the contrary, if the channel is occupied in the first CCA slot, the communication node selects N (N∈{1, 2, . . . , q}) arbitrarily (i.e., randomly) and stores the selected N value as an initial count. Then, the communication node senses a channel state on a CCA slot basis. Each time the channel is unoccupied in one specific CCA slot, the communication node decrements the count by 1. If the count is 0, the communication node may transmit data by securing a time period of up to (13/32)q ms.

3.3 Discontinuous Transmission in DL

When discontinuous transmission is performed on an unlicensed carrier having a limited maximum transmission period, the discontinuous transmission may influence on several functions necessary for performing an operation of LTE system. The several functions can be supported by one or more signals transmitted at a starting part of discontinuous LAA DL transmission. The functions supported by the signals include such a function as AGC configuration, channel reservation, and the like.

When a signal is transmitted by an LAA node, channel reservation has a meaning of transmitting signals via channels, which are occupied to transmit a signal to other nodes, after channel access is performed via a successful LBT operation.

The functions, which are supported by one or more signals necessary for performing an LAA operation including discontinuous DL transmission, include a function for detecting LAA DL transmission transmitted by a UE and a function for synchronizing frequency and time. In this case, the requirement of the functions does not mean that other available functions are excluded. The functions can be supported by other methods.

3.3.1 Time and Frequency Synchronization

A design target recommended by LAA system is to support a UE to make the UE obtain time and frequency synchronization via a discovery signal for measuring RRM (radio resource management) and each of reference signals included in DL transmission bursts, or a combination thereof. The discovery signal for measuring RRM transmitted from a serving cell can be used for obtaining coarse time or frequency synchronization.

3.3.2 DL Transmission Timing

When a DL LAA is designed, it may follow a CA timing relation between serving cells combined by CA, which is defined in LTE-A system (Rel-12 or earlier), for subframe boundary adjustment. Yet, it does not mean that a base station starts DL transmission only at a subframe boundary. Although all OFDM symbols are unavailable in a subframe, LAA system can support PDSCH transmission according to a result of an LBT operation. In this case, it is required to support transmission of control information necessary for performing the PDSCH transmission.

3.4 Measuring and Reporting RRM

LTE-A system can transmit a discovery signal at a start point for supporting RRM functions including a function for detecting a cell. In this case, the discovery signal can be referred to as a discovery reference signal (DRS). In order to support the RRM functions for LAA, the discovery signal of the LTE-A system and transmission/reception functions of the discovery signal can be applied in a manner of being changed.

3.4.1 Discovery Reference Signal (DRS)

A DRS of LTE-A system is designed to support on/off operations of a small cell. In this case, off small cells correspond to a state that most of functions are turned off except a periodic transmission of a DRS. DRSs are transmitted at a DRS transmission occasion with a period of 40, 80, or 160 ms. A DMTC (discovery measurement timing configuration) corresponds to a time period capable of anticipating a DRS received by a UE. The DRS transmission occasion may occur at any point in the DMTC. A UE can anticipate that a DRS is continuously transmitted from a cell allocated to the UE with a corresponding interval.

If a DRS of LTE-A system is used in LAA system, it may bring new constraints. For example, although transmission of a DRS such as a very short control transmission without LBT can be permitted in several regions, a short control transmission without LBT is not permitted in other several regions. Hence, a DRS transmission in the LAA system may become a target of LBT.

When a DRS is transmitted, if LBT is applied to the DRS, similar to a DRS transmitted in LTE-A system, the DRS may not be transmitted by a periodic scheme. In particular, it may consider two schemes described in the following to transmit a DRS in the LAA system.

As a first scheme, a DRS is transmitted at a fixed position only in a DMTC configured on the basis of a condition of LBT.

As a second scheme, a DRS transmission is permitted at one or more different time positions in a DMTC configured on the basis of a condition of LBT.

As a different aspect of the second scheme, the number of time positions can be restricted to one time position in a subframe. If it is more profitable, DRS transmission can be permitted at the outside of a configured DMTC as well as DRS transmission performed in the DMTC.

FIG. 26 is a diagram for explaining DRS transmission methods supported by LAA system.

Referring to FIG. 26, the upper part of FIG. 26 shows the aforementioned first scheme for transmitting a DRS and the bottom part of FIG. 10 shows the aforementioned second scheme for transmitting a DRS. In particular, in case of the first scheme, a UE can receive a DRS at a position determined in a DMTC period only. On the contrary, in case of the second scheme, a UE can receive a DRS at a random position in a DMTC period.

In LTE-A system, when a UE performs RRM measurement based on DRS transmission, the UE can perform single RRM measurement based on a plurality of DRS occasions. In case of using a DRS in LAA system, due to the constraint of LBT, it is difficult to guarantee that the DRS is transmitted at a specific position. Even though a DRS is not actually transmitted from a base station, if a UE assumes that the DRS exists, quality of an RRM measurement result reported by the UE can be deteriorated. Hence, when LAA DRS is designed, it is necessary to permit the existence of a DRS to be detected in a single DRS occasion. By doing so, it may be able to make the UE combine the existence of the DRS with RRM measurement, which is performed on successfully detected DRS occasions only.

Signals including a DRS do not guarantee DRS transmissions adjacent in time. In particular, if there is no data transmission in subframes accompanied with a DRS, there may exist OFDM symbols in which a physical signal is not transmitted. While operating in an unlicensed band, other nodes may sense that a corresponding channel is in an idle state during a silence period between DRS transmissions. In order to avoid the abovementioned problem, it is preferable that transmission bursts including a DRS signal are configured by adjacent OFDM symbols in which several signals are transmitted.

3.5 Channel Access Procedure and Contention Window Adjustment Procedure

In the following, the aforementioned channel access procedure and the contention window adjustment procedure are explained in the aspect of a transmission node.

FIG. 27 is a flowchart for explaining CAP and CWA.

In order for an LTE transmission node (e.g., a base station) to operate in LAA Scell(s) corresponding to an unlicensed band cell for DL transmission, it may initiate a channel access procedure (CAP) [S2710].

The base station can randomly select a back-off counter N in a contention window (CW). In this case, the N is configured by an initial value Ninit [S2720]. The Ninit is randomly selected from among values ranging from 0 to CW_(p).

Subsequently, if the back-off counter value (N) corresponds to 0 [S2722], the base station terminates the CAP and performs Tx burst transmission including PSCH [S2724]. On the contrary, if the back-off value is not 0, the base station reduces the back-off counter value by 1 [S2730].

The base station checks whether or not a channel of the LAA Scell(s) is in an idle state. If the channel is in the idle state, the base station checks whether or not the back-off value corresponds to 0 [S2750]. The base station repeatedly checks whether or not the channel is in the idle state until the back-off value becomes 0 while reducing the back-off counter value by 1.

In the step S2740, if the channel is not in the idle state i.e., if the channel is in a busy state, the base station checks whether or not the channel is in the idle state during a defer duration (more than 15 usec) longer than a slot duration (e.g., 9 usec) [S2742]. If the channel is in the idle state during the defer duration, the base station can resume the CAP [S2744]. For example, when the back-off counter value Ninit corresponds to 10, if the channel state is determined as busy after the back-off counter value is reduced to 5, the base station senses the channel during the defer duration and determines whether or not the channel is in the idle state. In this case, if the channel is in the idle state during the defer duration, the base station performs the CAP again from the back-off counter value 5 (or, from the back-off counter value 4 by reducing the value by 1) rather than configures the back-off counter value Ninit. On the contrary, if the channel is in the busy state during the defer duration, the base station performs the step S2742 again to check whether or not the channel is in the idle state during a new defer duration.

Referring back to FIG. 27, the base station checks whether or not the back-off counter value (N) becomes 0 [S2750]. If the back-off counter value (N) becomes 0, the base station terminates the CAP and may be able to transmit a Tx burst including PDSCH.

The base station can receive HARQ-ACK information from a UE in response to the Tx burst [S2770]. The base station can adjust a CWS (contention window size) based on the HARQ-ACK information received from the UE [S2780].

In the step S2780, as a method of adjusting the CWS, the base station can adjust the CWS based on HARQ-ACK information on a first subframe of a most recently transmitted Tx burst (i.e., a start subframe of the Tx burst).

In this case, the base station can set an initial CW to each priority class before the CWP is performed. Subsequently, if a probability that HARQ-ACK values corresponding to PDSCH transmitted in a reference subframe are determined as NACK is equal to or greater than 80%, the base station increases CW values set to each priority class to a next higher priority.

In the step S2760, PDSCH can be assigned by a self-carrier scheduling scheme or a cross-carrier scheduling scheme. If the PDSCH is assigned by the self-carrier scheduling scheme, the base station counts DTX, NACK/DTX, or ANY state among the HARQ-ACK information fed back by the UE as NACK. If the PDSCH is assigned by the cross-carrier scheduling scheme, the base station counts the NACK/DTX and the ANY states as NACK and does not count the DTX state as NACK among the HARQ-ACK information fed back by the UE.

If bundling is performed over M (M>=2) number of subframes and bundled HARQ-ACK information is received, the base station may consider the bundled HARQ-ACK information as M number of HARQ-ACK responses. In this case, it is preferable that a reference subframe is included in the M number of bundled subframes.

4. Proposed Embodiments

4.1 Method of Transmitting and Receiving Tracking Reference Signal in LAA System

FIGS. 28 to 30 are diagrams illustrating a method of transmitting and receiving a TRS (tracking reference signal) according to a proposed embodiment.

A base station can perform LBT to transmit data or discontinuously transmit data for a different reason on an unlicensed band. The discontinuous data transmission influences on reception capability of a data channel (e.g., PDSCH) and/or a control channel (e.g., (E)PDCCH) of a user equipment. In particular, in order for the user equipment to receive a specific downlink channel, it is necessary for the user equipment to configure a coefficient of a filter for estimating a channel based on a result of estimating such a parameter as power delay profile, delay spread and Doppler spectrum, Doppler spread, etc. And, it is necessary for the user equipment to prepare synchronization and tracking based on a result of estimating a time/frequency error. The user equipment can successfully receive a downlink channel only when the preparation is completed using the abovementioned parameters. When a channel of an unlicensed band is in an idle state at specific timing, although a base station performs an LBT procedure and transmits a downlink channel to a user equipment, if a certain work for receiving the downlink channel is not preferentially performed, the user equipment may fail to receive the downlink channel.

Hence, synchronization and tracking requirements for receiving a downlink channel can be defined for a user equipment. For example, if the number of tracking reference signals (TRSs) received during a specific time period or a time window is equal to or greater than a prescribed number, a base station and a user equipment can define it as a requirement for tracking performance of the user equipment is satisfied. The requirement can be randomly configured. The requirement can be differently determined according to a modulation order of data included in a downlink channel to be received by the user equipment.

In the following embodiment, when a tracking requirement is defined, a method of transmitting and receiving a TRS for satisfying the requirement is proposed.

As mentioned in the foregoing description, in a legacy LTE/LTE-A system, a DRS is utilized to discover a cell which is deactivated due to no data traffic. The DRS is configured by a CSI-RS (optional) in addition to a PSS/SSS and a CRS. A base station periodically sets DMTC to a user equipment in a unit of 6 ms and transmits a DRS within the DMTC. The user equipment identifies a cell and performs RRM by receiving the DRS within the set DMTC. Meanwhile, according to the proposed embodiment, the base station and the user equipment can utilize the DRS to perform tracking. In particular, the base station and the user equipment utilize the DRS as a TRS to satisfy an insufficient tracking requirement.

Meanwhile, when data to be transmitted in a manner of being offloaded to an unlicensed band are intensively arrived, if a DMTC section is configured too often, although there is no data to be transmitted, since a DRS is frequently transmitted, it could be inefficient. Moreover, if the DMTC section is frequently configured, interference affecting a different communication system (e.g., WiFi system or inter-operator LAA system) coexisted on an unlicensed band increases. On the contrary, if the DMTC section is configured to be too long, since a time section for transmitting a DRS is reduced, RSs for performing synchronization and tracking become insufficient. As a result, a probability of failing to satisfy a requirement for tracking performance increases.

In order to solve the abovementioned problem, as shown in FIG. 28, a base station can configure DMTC of two types. For example, a primary DMTC corresponds to a time section having a relatively long period and a secondary DMTC corresponds to a time section having a relatively short period. In particular, in FIG. 28, such a relation as T1>T2 is satisfied. According to one embodiment, a base station always performs LBT during the primary DMTC to transmit a DRS. If it is determined that it is necessary to transmit a DRS (e.g., if it is determined that a tracking requirement is not satisfied), the base station selectively performs LBT during the secondary DMTC to transmit the DRS. If it is determined that a tracking requirement required by a user equipment is satisfied, the base station may omit transmission of a DRS during the secondary DMTC [2810]. In particular, DRS transmission via the secondary DMTC can be configured in advance or can be restrictively configured only when the DRS transmission is necessary. The base station can explicitly inform the user equipment of whether or not a DRS is transmitted via the secondary DMTC using DCI.

The primary DMTC can be set to the base station using a scheme identical or similar to a legacy LTE/LTE-A system. Meanwhile, the base station can set the secondary DMTC to the user equipment via higher layer signaling. The base station can set both the primary DMTC and the secondary DMTC to the user equipment. And, the base station may commonly set the secondary DMTC to user equipments belonging to a cell. Moreover, a section where the primary DMTC and the secondary DMTC are overlapped can be regarded as the primary DMTC. A procedure of measuring a neighboring cell can be performed during the primary DMTC only.

FIGS. 29 and 30 are flowcharts illustrating a different embodiment for a method of transmitting and receiving a TRS. FIG. 29 illustrates a procedure of transmitting a TRS in the aspect of a base station and FIG. 30 illustrates a procedure of receiving a TRS in the aspect of a user equipment, respectively.

First of all, the base station can utilize other signals rather than a DRS as a TRS. This is because it is not easy to satisfy a tracking requirement with a DRS transmitted in a unit of scores of ms in LTE/LTE-A system. For example, a DMRS or a preamble indicating the start of downlink transmission can be utilized as a TRS. In particular, in addition to a DRS (PSS, SSS, CSI-RS), a DMRS and a DL preamble can be utilized as a TRS. The base station transmits the DMRS and the DL preamble to the user equipment to achieve a requirement satisfying tracking performance.

Specifically, the base station determines that a tracking performance requirement of the user equipment intending to transmit downlink data is not satisfied [S2910]. If tracking performance is not sufficient, it is necessary for the base station to preferentially transmit a TRS to the user equipment before downlink data is transmitted to the user equipment. Hence, the base station transmits the TRS to the user equipment in a subframe in which a DRS is not transmitted. Meanwhile, since the user equipment may be unaware of a fact that the base station transmits a DMRS and a DL preamble as a TRS, the base station informs the user equipment of the transmission of a TRS via UE-specific DCI in advance [S2920]. The DCI can be transmitted to the user equipment via a Pcell and/or UScell (Scell of unlicensed band). For example, the base station changes at least one selected from the group consisting of a scrambling sequence of the DCI, a CRS mask, and a search space or adds a new indicator to the DCI to inform the user equipment that the TRS is transmitted in a corresponding subframe. In particular, a scheme of informing each user equipment of TRS transmission has a merit in that it is able to reuse a DCI format compared to a scheme of commonly informing all user equipments belonging to cell coverage of TRS transmission via common DCI of Pcell. On the contrary, if there are many user equipments requiring a TRS, it is more efficient to use a scheme that the base station commonly assigns an RNTI value to user equipments connected with the base station and informs the user equipments of transmission of a TRS using the RNTI value.

Having received a signal indicating transmission of a TRS from the base station, the user equipment performs tracking using the received TRS in a corresponding subframe. In particular, the user equipment utilizes not only a DRS received in the subframe but also a DMRS and/or a DL preamble as the TRS [S2930]. As a result, although it is not sufficient with the DRS only, the user equipment can satisfy a tracking requirement.

Unlike the aforementioned embodiment, the user equipment may autonomously determine that the tracking requirement is not satisfied. If the user equipment not satisfied with the tracking requirement receives DCI indicating transmission of a TRS from the base station [S2920], the user equipment does not attempt to demodulate PDSCH scheduled in a corresponding subframe and utilizes the subframe for a tracking use only [S2930]. In this case, the user equipment may utilize a scheduled RB of the subframe or RSs belonging to the entire RBs as TRSs. If the tracking requirement is not satisfied, although downlink data transmission is scheduled, the user equipment does not transmit HARQ-ACK/NACK to the base station and the base station does not expect HARQ-ACK/NACK from the user equipment.

Meanwhile, although the base station transmits TRSs to the user equipment as many as TRSs satisfying tracking performance of the user equipment, the user equipment may fails to receive a part of RSs transmitted by the base station. For example, if there exists a hidden node near the user equipment, the user equipment may determine that a tracking requirement is not satisfied despite of the transmission of the base station. In this case, a mismatch problem that the number of TRSs transmitted to the user equipment in the aspect of the base station is not matched with the number of TRSs received from the base station in the aspect of the user equipment may occur. As a result, tracking performance of the user equipment is not sufficiently satisfied [S3010].

Since the base station does not know the status of the user equipment, the base station determines that the tracking performance of the user equipment is satisfied and schedules general downlink data rather than transmits a TRS [S3020]. Yet, since the tracking performance is not satisfied, the user equipment fails to successfully receive the downlink data and utilizes a subframe as a tracking use only [S3030]. Or, irrespective of whether or not a tracking requirement is satisfied in the aspect of transmission of the base station, if a tracking requirement is not guaranteed in the aspect of reception of the user equipment, the same result may occur. In particular, if a tracking requirement is not satisfied in the aspect of reception, the user equipment utilizes a corresponding subframe for a tracking use only rather than processes downlink data.

In this case, if PDSCH scheduled in a received subframe corresponds to a firstly transmitted data, the user equipment may not feedback HARQ ACK/NACK to the base station [S3040]. Or, the user equipment may feedback ‘DTX’ to the base station to initialize an RV (redundancy version) value. Or, the user equipment may not perform buffering on the received data.

If PDSCH scheduled in the received subframe corresponds to a retransmitted data, the user equipment may not feedback HARQ ACK/NACK to the base station or feedback ‘DTX’ to the base station to maintain an RV value. Or, the user equipment may feedback NACK to the base station to prevent an aggregation level from being excessively higher [S3050]. Or, the user equipment may not perform combining between data received in a previous subframe and data previously received.

Meanwhile, a user equipment not satisfied with a TRS requirement can detect not only PDSCH scheduled to the user equipment but also PDSCH transmitted to a different user equipment via blind detection. In particular, the user equipment may utilize an RS or a DL preamble included in PDSCH of a neighboring user equipment for a tracking procedure of the user equipment [S3060].

Meanwhile, if a user equipment determines that a tracking requirement is not satisfied, the user equipment may directly ask a base station to transmit a TRS [S3070]. For example, the user equipment can periodically receive a PUCCH resource for requesting TRS transmission from the base station via a procedure similar to a scheduling request. If a TRS is additionally required, the user equipment may ask the base station to transmit a TRS using a resource allocated in advance. As a different example, if a TRS transmission request is triggered by receiving DCI from the base station, the user equipment may inform the base station of whether or not TRS transmission is necessary via a resource mapped to the DCI.

In particular, the user equipment can satisfy a tracking requirement of the user equipment via various methods. The abovementioned embodiments can be implemented independently or in a manner of being combined with a different example. Moreover, the embodiments are just examples for satisfying a tracking requirement of the user equipment. Unmentioned various examples can be utilized for transmitting and receiving a TRS and achieving a tracking requirement. And, the base station can inform the user equipment of information on whether or not the embodiments are applied and detail information on a parameter and a rule via physical layer signaling or higher layer signal.

4.2 RRM Measurement Method in LAA System

FIG. 31 is a flowchart for a method of performing RRM (radio resource measurement) according to a proposed embodiment.

In small cell environment of a legacy LTE system, a DRS is utilized for performing RRM and is transmitted to a user equipment within a DMTC section of 6 ms unit to discover a cell which is deactivated (or, turned off) due to no traffic. The user equipment performs cell identification, RRM, and the like using the DRS received within the DMTC section.

Meanwhile, since LBT is performed on an unlicensed band of LAA system, although a serving cell is activated, it is difficult to always guarantee downlink transmission in the serving cell. In particular, as mentioned earlier in the paragraph 3.4, the LAA system basically supports RRM measurement via a DRS. Yet, since a base station performs an LBT operation not only for transmission of downlink data but also for transmission of the DRS, it may be difficult to always guarantee the transmission of the DRS for performing RRM.

In the following, when the transmission of the DRS is not sufficiently guaranteed due to the LBT, a method of performing RRM and a method of reporting the RRM are proposed.

Basically, a base station sets a period of reporting an RRM measurement result to a user equipment [S3110]. The user equipment performs RRM measurement using samples capable of performing the RRM measurement during the period set to the user equipment, applies L1/L3 filtering to a measurement result, and reports the final result value to the base station.

In this case, if the base station fails to perform LBT, the base station may not transmit a DRS to the user equipment. Or, a DRS transmitted by the base station may not be successfully received by the user equipment. In this case, the user equipment determines that measurement samples obtained during a prescribed period are not sufficient [S3120]. If the minimum number of measurement samples required for reporting such a measurement result as RSRP, RSSI, RSRQ, and the like is defined, an RRM result measured by the insufficient measurement samples is incorrect.

In the following, a proposed embodiment for a procedure of measuring and reporting RRM using the insufficient number of samples is explained. First of all, if a user equipment performs RRM measurement using insufficient samples, the user equipment may not report a result of the measurement to a base station. On the contrary, when the user equipment reports a measurement result to the base station, the user equipment can also inform the base station that the measurement is performed via insufficient samples [S3130]. For example, the user equipment can inform the base station of a measurement result together with a fact that the measurement result is measured from insufficient samples. Or, the user equipment may inform the base station of only information indicating that the samples were not sufficient using a predefined value. Or, the user equipment may report a measurement result calculated via a different method (e.g., a mode value, a median value, or a maximum value) to the base station without applying L1/L3 filtering. In case of reporting a measurement result measured using a different method instead of the L1/L3 filtering, the user equipment can also inform the base station that the measurement is performed via insufficient samples.

Meanwhile, irrespective of a case that the base station fails to transmit the sufficient number of DRSs, although the base station transmits the sufficient number of DRSs to the user equipment, the user equipment may fail to receive the sufficient number of DRSs transmitted by the base station due to a hidden node problem and the like. In this case, a mismatch problem occurs between timing capable of performing RRM measurement in the aspect of the base station and timing capable of performing RRM measurement in the aspect of the user equipment.

In order to solve the mismatch problem, when the user equipment reports an RRM measurement result to the base station, the user equipment can also report time information (e.g., timestamp information or log information) indicating the timing at which the measurement is performed to the base station. Yet, if the user equipment informs the base station of the time information together with the measurement result, it may become signaling overhead. And, since all user equipments belonging to a serving cell perform measurement and report a measurement result, it may lead to a result that excessive resources are required for reporting the measurement result.

In order to prevent occurrence of a problem while the aforementioned mismatch problem is solved, the user equipment can additionally report values described in the following to the base station in addition to a result value to which the L1/L3 filtering is applied. For example, the user equipment can inform the base station of information indicating the number of samples (e.g., the number of DRSs, the number of subframes, the number of OFDM symbols, etc.) on which measurement is performed. Or, the user equipment can inform the base station of information on at least one selected from the group consisting of a maximum value, a minimum value, a mode value, and a median value among values measured during a period of reporting a measurement result.

In a situation that the sufficient number of samples required for performing RRM measurement is not secured, the user equipment can inform the base station that a measurement result is incorrect by forwarding additional information to the base station. The abovementioned embodiments can be implemented independently or in a manner of being combined with a different example. Moreover, the embodiments are just examples for performing measurement and reporting a measurement result. Unmentioned various examples can be utilized in a similar form. And, the base station can inform the user equipment of information on whether or not the embodiments are applied and detail information on a parameter and a rule via physical layer signaling or higher layer signal.

5. Device Configuration

FIG. 32 is a diagram illustrating configurations of a UE and an eNB capable of being implemented by the embodiments proposed in the present invention. The UE and the eNB shown in FIG. 32 operate to implement embodiments of a method of transmitting and receiving a tracking reference signal and a method of performing RRM measurement.

A UE may act as a transmission end on a UL and as a reception end on a DL. An eNB may act as a reception end on a UL and as a transmission end on a DL.

That is, each of the UE and the eNB may include a Transmitter (Tx) 3240 or 3250 and a Receiver (Rx) 3260 or 3270, for controlling transmission and reception of information, data, and/or messages, and an antenna 3200 or 3210 for transmitting and receiving information, data, and/or messages.

Each of the UE and the eNB may further include a processor 32820 or 3230 for implementing the afore-described embodiments of the present disclosure and a memory 3280 or 3290 for temporarily or permanently storing operations of the processor 32820 or 3230.

The Tx and Rx of the UE and the eNB may perform a packet modulation/demodulation function for data transmission, a high-speed packet channel coding function, OFDM packet scheduling, TDD packet scheduling, and/or channelization. Each of the UE and the eNB of FIG. 32 may further include a low-power Radio Frequency (RF)/Intermediate Frequency (IF) module.

Meanwhile, the UE may be any of a Personal Digital Assistant (PDA), a cellular phone, a Personal Communication Service (PCS) phone, a Global System for Mobile (GSM) phone, a Wideband Code Division Multiple Access (WCDMA) phone, a Mobile Broadband System (MBS) phone, a hand-held PC, a laptop PC, a smart phone, a Multi Mode-Multi Band (MM-MB) terminal, etc.

The smart phone is a terminal taking the advantages of both a mobile phone and a PDA. It incorporates the functions of a PDA, that is, scheduling and data communications such as fax transmission and reception and Internet connection into a mobile phone. The MB-MM terminal refers to a terminal which has a multi-modem chip built therein and which can operate in any of a mobile Internet system and other mobile communication systems (e.g. CDMA 2000, WCDMA, etc.).

Embodiments of the present disclosure may be achieved by various means, for example, hardware, firmware, software, or a combination thereof.

In a hardware configuration, the methods according to exemplary embodiments of the present disclosure may be achieved by one or more Application Specific Integrated Circuits (ASICs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers, microcontrollers, microprocessors, etc.

In a firmware or software configuration, the methods according to the embodiments of the present disclosure may be implemented in the form of a module, a procedure, a function, etc. performing the above-described functions or operations. A software code may be stored in the memory 3280 or 3290 and executed by the processor 3220 or 3230. The memory is located at the interior or exterior of the processor and may transmit and receive data to and from the processor via various known means.

Those skilled in the art will appreciate that the present disclosure may be carried out in other specific ways than those set forth herein without departing from the spirit and essential characteristics of the present disclosure. The above embodiments are therefore to be construed in all aspects as illustrative and not restrictive. The scope of the disclosure should be determined by the appended claims and their legal equivalents, not by the above description, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein. It is obvious to those skilled in the art that claims that are not explicitly cited in each other in the appended claims may be presented in combination as an embodiment of the present disclosure or included as a new claim by a subsequent amendment after the application is filed.

INDUSTRIAL APPLICABILITY

The present disclosure is applicable to various wireless access systems including a 3GPP system, and/or a 3GPP2 system. Besides these wireless access systems, the embodiments of the present disclosure are applicable to all technical fields in which the wireless access systems find their applications. Moreover, the proposed method can also be applied to mmWave communication using an ultra-high frequency band. 

What is claimed is:
 1. A method of receiving a TRS (tracking reference signal), which is received by a user equipment (UE) in a wireless communication system supporting an unlicensed band, the method comprising: receiving, from a base station, a first TRS transmitted in a first subframe, wherein the first TRS is a DRS (discovery reference signal); receiving downlink control information (DCI) scheduling a data channel transmitted in a second subframe, wherein the DCI further indicates that a second TRS is transmitted in the second subframe; receiving the second TRS transmitted in the second subframe based on the DCI; determining a TRS requirement being met, wherein the TRS requirement is determined to be met when a number of the first TRS received during a specific duration is equal to or more than a prescribed number, and wherein the scheduled data channel transmitted in the second subframe is not demodulated or an ACK/NACK (acknowledgment/negative ACK) for the scheduled data channel is not transmitted when the TRS requirement is determined not to be met; and performing tracking using the first TRS, wherein the performing tracking further uses the second TRS when the TRS requirement is determined not to be met.
 2. The method of claim 1, wherein the second TRS includes a DMRS (demodulation reference signal), a CRS (cell-specific reference signal), a CSI-RS (channel state information reference signal) and a downlink transmission preamble transmitted.
 3. The method of claim 1, wherein the DCI comprises a UE-specifically transmitted DCI or an RNTI (radio network temporary identifier) commonly assigned to UE connected with the base station.
 4. The method of claim 1, when the TRS requirement is not met, further comprising requesting of TRS transmission to the base station.
 5. The method of claim 1, wherein when the TRS requirement is not met, the first subframe comprises a secondary DMTC (discovery measurement timing configuration) configured with a period shorter than a period of a primary DMTC.
 6. A user equipment (UE) receiving a TRS (tracking reference signal) in a wireless communication system supporting an unlicensed band, the user equipment comprising: a transmitter; a receiver; and a processor configured to operate in a manner of being connected with the transmitter and the receiver, the processor receives, from a base station, a first TRS transmitted in a first subframe, wherein the first TRS is a DRS (discovery reference signal), the processor receives downlink control information (DCI) scheduling a data channel transmitted in a second subframe, wherein the DCI further indicates that a second TRS is transmitted in the second subframe, the processor receives the second TRS transmitted in the second subframe based on the DCI; the processor determines a TRS requirement being met, wherein the TRS requirement is determined to be met when a number of the first TRS received during a specific duration is equal to or more than a prescribed number, and wherein the scheduled data channel transmitted in the second subframe is not demodulated or an ACK/NACK (acknowledgment/negative ACK) for the scheduled data channel is not transmitted when the TRS requirement is determined not to be met; and the processor performs tracking using the first TRS, wherein the performing tracking further uses the second TRS when the TRS requirement is determined not to be met.
 7. The user equipment of claim 6, wherein the second TRS includes a DMRS (demodulation reference signal), a CRS (cell-specific reference signal), a CSI-RS (channel state information reference signal) and a downlink transmission preamble transmitted.
 8. The user equipment of claim 6, wherein the DCI comprises a UE-specifically transmitted DCI or an RNTI (radio network temporary identifier) commonly assigned to UE connected with the base station.
 9. The user equipment of claim 6, wherein when the TRS requirement is not met, further comprising requesting of TRS transmission to the base station.
 10. The user equipment of claim 6, wherein when the TRS requirement is not met, the first subframe comprises a secondary DMTC (discovery measurement timing configuration) configured with a period shorter than a period of a primary DMTC. 